Dog Days ....

The expression originated in Roman times as canicularis dies, "days of the dog," and was an astronomical expression referring to the dog star Sirius, or possibly Procyon. The Romans linked the rising of the Dog Star, the most brilliant star in the constellation, Canis Major, with the sultry summer heat, believing that the star added to the extreme heat of the sun. "Canicular days," of course, have nothing to do with heat from the Dog Star, but the ancient expression remains popular after more than 20 centuries.

Dogs aren't the prized, often pampered pets in other countries that they are in America. In the East they are often considered pariahs, scavengers of the streets, and the Chinese, Koreans, and Japanese, among other Asians, commonly eat them.
Englishmen of earlier times used dogs primarily for hunting and kept them outside in a rude shelter, not generally as house pets. The dogs were fed table scraps there wasn't any further use for, and these they had to fight over. It didn't seem ideal, a dogs's life, and Englishmen of the 16th century began to compare anyone who had become impoverished, who was going to utter ruin naturally or morally, to their maltreated canines.

To lead a dog's life was to be bothered every moment, never to be left in peace; to go to the dogs was to become just like the helpless animals; and to die like a dog was to come to a miserable, shameful end. There were many other similar phrases that arose before the dogs had their day in England and America, including throw it to the dog, sick like a dog and dirty dog.

Snake in the grass

Latet anguis in herbe ("a snake lurks in the grass"), the Roman Poet Virgil wrote in the third Eclogue, and from this ancient source comes our common expression for a hidden or hypocritical enemy.
Proving that times don't change much, the Latin proverb first appears in English as a line in a political song of about 1920:
"Though all appears clean, a snake lurks in the grass."

Dr. Livingstone, I presume?

The famous greeting was put by Sir Henry Morton Stanley, the British explorer and journalist, to the explorer and missionary Dr. David Livingstone at Ujiji, Lake Tanganyika on 10 November 1871.
Stanley had been sent by the New York Herald to look for Livingstone who was missing on a journey in central Africa. In How I Found Livingstone (1872), Stanley described the moment:
'I would have run to him, only I was a coward in the presence of such a mob - would have embraced him, only, him being an Englishman, I did not know how he would receive me; so I did what cowardice and false pride suggested was the best thing - walked deliberately to him, took off my hat and said:

"Dr. Livingstone, I presume?"
"Yes," said he, with a kind smile, lifting his cap slightly.'

Pull out all the stops

An organist who pulled out all the stops - levers used to change the sound of the instrument - would be giving his all, the meaning of this phrase. Though the organ, or its rude prototype, originated in Greece, the expression deriving from it dates back to 19th-century England.

On the Q.T.

A British broadside ballad (1870) contained the line "Whatever I tell you is on the Q.T." This is the first record of Q.T. for 'on the quiet, in confidence' recorded in English, but no one has established whether the broad-side's anonymous author was the first person to use the initials Q.T. to stand for quiet.
On the Q.T. gained more popularity when it appeared in an 1891 minstrel show number called "Ta-ra-ra-boom-de-ay." London went 'stark mad with the refrain,' which was written by Henry J. Sayers and sung by Lottie Collins.
The first stanza follows:

A sweet Tuxedo girl you see,
Queen of swell society,
Fond of fun as fun can be
When it's on the strict Q.T.
I'm not too young, I'm not too old,
Not too timid, not too bold,
Just the kind of sport I'm told -
Ta-ra-ra-boom-de-ay....

"Ta Ra Ra Bum De Ay"

Drop in the bucket

Another biblical phrase, meaning very little compared with the whole.
It is from Isaiah 40:15

"Behold, the nations are as a drop in the bucket,
and are counted as the small dust on the balance."

Pot Luck

No sirloins or barons of beef for peasants in the Middle Ages. Mostly their dinners came from the great iron pots simmering over their fires into which leftovers were tossed from day to day. Often they did not know exactly what they were having for dinner, so when they asked a visitor to take pot luck with them they were not trying to put him off. It was a matter of luck - what was in the port and whether there would be enough of it to go around.
The expression is first recorded in 1592 and came to mean 'plain fare,' nothing fancy, what we usually have, like the French pot-au-feu ("fire-pot"), the ordinary family dinner. In Ireland the pot of hospitality always hung over the open fire ready to be dipped into by any unexpected visitor.
People who offer visitors pot luck today, however, often wind up preparing an impromptu banquet for their guests.

Here's mud in your eye...

This toast was originally made in the muddy trenches of World War I,
or in the cafes where English and American soldiers
spent their leaves trying to forget them.

Scrooge

Old Scrooge in Dickens's A Christmas Carol is not given any credit in the language for eventually becoming a genial old chap at the tale's end; his name still means a miserly, mean old man (and, sometimes, woman). Incidentally, Scrooge may well be a real English name. Dickens, like Balzac, was in the habit of collecting real names to use in his stories. As Joseph Shipley has pointed out, Scrooge is an apt name for the character, suggesting someone always ready "to put the screws on."

Keep your shirt on!

The stiff, starched shirts worn by American men back in the mid-19th century when this expression originated weren't made for a man to fight in. Therefore, men often removed their shirts when enraged and ready to fight, a practice that is reflected in the older British expression to get one's shirt out, "to lose one's temper." Keep your shirt on was a natural admonition from someone who didn't want to fight and realized that an argument could be settled if both parties kept calm and collected.

Received with thanks from Bruno Watson

The Full Monty

(more properly 'the Full Monte')

This expression means the real thing. It originates from the Monte Carlo Car Rally.
It seems that in the early days of the rally, the day before the race itself, the gentlemen of Monte Carlo were allowed to drive around the marked circuit in their own sports cars.
This practice was known locally as 'the Half Monte' ie. a practice.
The 'Full Monte' was therefore...the real thing.

Bite the dust

Every time we hear of still another desperado biting the dust in Western films, we are hearing an almost literal translation of a line found in Homer's Iliad, written thousands of years ago. American poet William Cullen Bryant translated the words in 1870: "...his fellow warriors, may a one, fall round him to the earth and bite the dust."

Earlier, Alexander Pope had eloquently translated the phrase as "bite the bloody sand" and English poet William Cowper hat it, literally, as "bite the ground."

The idea remains the same in any case: a man falling dead in combat, biting the dust in his last hostile, futile act.

Tie the Knot

In ancient times the marriage ceremony in many parts of the world consisted only of a priest or the family patriarch knotting together the garments of the bride and the groom to symbolize a permanent union. The practice, still a custom in some countries today, is the basis for the universal saying to tie the knot, meaning to get married, for which tying the knot has been a symbol in England since at least 1275.

The Greeks followed the custom of untying the knot to declare a marriage. Brides used the Herculean knot, a representation of the snakes entwined on the rod of Mercury, to fasten their woolen girdles. Only the bridegroom was allowed to untie this knot, praying as he did so that the gods would make his marriage as fruitful as that of Hercules - that is, very fruitful indeed, for Hercules once married the 50 daughters of Thestius, all of whom gave birth to his children on the same night. This last was not one of the legendary 12 labors of Hercules.

Dead as a doornail

Dead as a doornail is an expression most of us learned first in Dickens's A Christmas Carol. Actually, it's much older than that, having appeared in the fourteenth-century Vision of Piers Plowman and in Shakespeare's Henry IV.
Until now all word experts have been explaining that the doornail referred to is the heavy stud in the middle of a wooden door against which a knocker is struck. Since this happens many thousands of times - with a well-exercised knocker, at any rate - the doornail may well be considered 'dead' from the abuse it takes.
Ha, ha, says William Wagner of Falls Church, Virginia. It's pretty obvious that you experts on words are not experts on carpentry. "The dictionary," he writes, "defines a doornail as 'a large-headed nail, easily clinched, for nailing doors, through the battens.' Now the 'clinching' makes the nail 'dead'. It cannot be easily withdrawn. 'Dead-nailing' is a term most any carpenter is familiar with. It is a technique frequently used in constructing doors for log cabins, construction shanties and the like - and it antedates the ready availability of screws and more sophisticated fastening devices. It would seem that you have gone somewhat far afield to explain a phrase derived from the simple action of bending the end of a nail to provide secure fastening."
Far afield indeed have we wandered. And thanks to you, Mr. Wagner, for spiking the old story, which we hereby label - you should forgive the expression - dead as a doornail. The dickens with it!

Not worth a damn
Not worth a tinkers's dam

Not worth a damn originally came from the common phrase 'not worth a tinker's dam,' this dam being a pellet of bread used by old-time tinkers to block small holes in pots and pans while they poured in solder to fix the leak. When the patch was secure, the dam was discarded.
So anything 'not worth a tinker's dam' was something utterly worthless.

O.K.

O.K. has probably been more discussed than any other item in the American language. Everyone from presidents to plumbers has his or her pet theory. Woodrow Wilson thought it was a Choctaw Indian word and should properly be spelled 'Okeh." He persuaded a record company of the 1920s (the one that made the first Louis Armstrong records) to call their product "Okey Records." But history fails to record that President Wilson converted many more people to his belief.
A distinguished Columbia professor, Allen Walker Read, announced in 1941 that the term originated as an abbreviation for the Old Kinderhook Club, a political organization supporting James Van Buren (The Kinderhook Fox) for the presidency in 1840. That theory was generally accepted until, in the mid 1960s, an equally distinguished scholar, Dr. Woodford A. Heflin, proved that O.K. had appeared in a Philadelphia newspaper in 1839 - a year ahead of Read's date. Professor Read then countered with evidence that a Boston paper had O.K. in print even earlier in 1839 - and there the scholarly argument rests.
But not everyone agrees. Charles Berlitz, the eminent linguist, thinks it may well come from 'Aux Cayes," a port in Haiti famous for its superior rum. This theory holds that American sailors were so enthusiastic about the rum that 'Aux Cayes' - later O.K. - became their expression for approval.
There are various other theories of the origins of O.K. The most popular holds that Andrew Jackson, while a court clerk in Tennessee, marked O.K. on legal ducuments as an abbreviation for the illiterate 'Orl Kerrect.' In fact, Jackson was never a court clerk - he was a prosecuting attorney - and he was far from illiterate, serving as representative and U.S. senator before being selected President. Mencken once called O.K. 'the most useful of Americanisms.'

It has ccertainly been successful in breeding theories about its origin.

Addition from: Joss, University of Wales Lampeter

I have my own theory for the origins of the expression 'O.K.'. Many settlers of America originated in Scotland, and it is my belief that O.K. is a corruption of 'Och Aye'.

Addition from: Dean G.

You forgot to include the oldest (and probably correct) origin of O.K. As many things, it comes from the Greek Language.
Ollos Kalla (all is good)...