Rule of Thumb
There are two good choices here. Brewmasters of old often tested the temperature of a batch of beer by dipping a thumb in the brew, their long experience telling them how well the beer was brewing. One theory has it that our expression for a rough, guesswork estimate derives from this practice.
More likely it stems from the ancient use of the last joint of the thumb as a measuring device for roughly one inch.
Additional information received with thanks from
Jayson E. D.
I've heard a couple of times from different sources that the "Rule of
Thumb" comes from an Old English law which forbade a man from beating
his wife with a rod "thicker than his thumb."
Additional information received with thanks from M.L. van Will
Tailors could actually make a suit of clothes for a
"properly-proportioned" man by applying the "rule of thumb".
I'm not
sure I remember it correctly and I don't have a string or tape-measure
handy but I believe it is as follows:
4 x around the thumb equals the wrist
2 x the wrist equals the neck
2 x the neck equals the waist
2 x the waist equals the span from finger-tip to finger-tip
Of course, if one is not perfectly proportioned, the suit might be a
strange fit.
Do I remember correctly (if vaguely) that this is how the
Lilliputians measured Gullivan for his new clothes?


ALL ROADS LEAD TO ROME
The ancient Romans built such an excellent system of roads that the saying arose all roads lead to Rome, that is, no matter which road one starts a journey on, he will finally reach Rome if he keeps on traveling. The popular saying came to mean that all ways or methods of doing something end in the same result, no method being better than another.


All is lost save honor
After Francis I of France was defeated by Spain's Charles V at Pavia, Italy in 1525, captured, and forced to sign a humiliating treaty, he sat down and wrote to his mother. His actual words were not so eloquent, but the most memorable phrase in his letter was translated into English as All is lost save honor. Despite the fact that Francis soon lost his honor by breaking the treaty, the sentiments of this patron of Rabelais and creator of Fontainbleau became proverbial.

COME HELL OR HIGH WATER
When we looked into the expression "come hell or high water," meaning that a person determined to accomplish a task, come hell or high water, would permit no obstacles to get in the way, we found that it originated as an expression of the folk wisdom of pioneers living on the frontier where the challenges of nature were everyday occurrances. We noted that it first appeared in print in the United States in 1915, but added that the expression had probably been around for many years before finding its way into print. A Sydney Sherry of Oconto, Wisconsin, adds, his recollection of the phrase. "Well, folks," he writes, " 'come hell or high water' was a very popular expression that as a child of ten years of age I heard in the gold fields of Ararat, Victoria, Australia, where I was born in 1905. It was an often used saying at the Calico (tent) School I attended. The expression probably came into Aussie slang from the Yankee diggers of those rip-roaring days. There were very few slang phrases used in the chopped-up King's English of Australia that had not originated in Yankee Land."
So it would appear that this colorful expression has been around for a good long time, if it had already reached Australia in the early years of this century.

Cat has nine lives...
...is one of the oldest English proverbs, being recorded in Heywood's collection in 1546 and being certainly much older than that date would indicate. The allusion is probably to the cat's legendary ability to land on all four feet when dropped or tossed from a height that would mean death to any other animal. Heywood's version goes like this: "No wife, no woman hath nine lives like a cat." And in Romeo and Juliet there is the following interchange between Tybalt and Mercutio: "What wouldst thou have with me?" asks Tybalt. "Good King of Cats, nothing but one of your nine lives," replies Mercutio.


"After me, the flood."apres moi le deluge
The popularity of the phrase stems from its use by Madame de Pompadour, celebrated beauty and intimate of King Louis XV of France. The French Court at the time was famed for its lavish and wasteful extravagances. When Pompadour, whose philosophy was "Live for the minute - who cares what happens when we're gone?" was reproved for these excesses, she replied, "Apres nous le deluge."


Everyone talks about the weather...
The most famous quotation on the subject of weather - "Everyone talks about the weather, but nobody does anything about it" - is usually attributed to Mark Twain. Maybe he did say it one time, but the best evidence indicated that his brother-in-law, Charles Dudley Warner, actually wrote it first in an editorial in the Hartford Courant. Perhaps, though, the whole matter may now be left aside and forgotten, for somebody has done something about the weather. His name was Willis H. Carrier and he invented air conditioning.

CROCODILE TEARS
The crocodile was a favorite figure in ancient Greek and Egyptian folklore. Indeed, its name comes directly from the Greek krokodeilos. The legend was that this giant reptile attracted its victims by loud moaning and then shed tears while it devoured them.


EUREKA!
There's quite a story involved here, with a star-studded cast of characters, the discovery of an important principle of physics, and even - as befits a tale told for today's broad-minded audiences - a scene in which the leading character disports himself in the nude.
It seems that King Hiero II of Syracuse (the Greek city, not the one in upstate New York) gave a certain amount of gold to an artisan for making a crown. When the crown was delivered, he suspected that some of the gold had been stolen and silver substituted. But how to prove it?
So he sent for his wisest philosopher, Archimedes, and turned the problem over to him. Archimedes, not knowing the solution, decided to relax and consider the problem while soaking in a tepid bath. As he climbed into the brimful tub, some of the water overflowed and Archimedes raced into the street, still in the buff, shouting:
"Eureka! Eureka!" - which is Greek for "I have found it!"
Archimedes discovered that his body, when immersed in water, was
subject to an upward force equal to the weight of water it displaced.
A gold crown and a silver crown will displace the same amount of water
but would of course have different weights. In other words, the
densities of the two crowns would be different. If the crown had a
certain proportion of silver and of gold, its density would be
somewhere in between. Archimedes' principle allows one to calculate
the proportion of silver and gold in the crown, by comparing the density
of the crown with the densities of pure gold and pure silver.
Archimedes proved that the goldsmith had indeed been cheating the king. Then, presumably, he put his clothes back on.
Incidentally, "Eureka"is the state motto of California, in reference to the discovery of gold by the forty-niners.

FLASH IN THE PAN
Flash in the pan is generally thought to have originated in the days of flintlock muskets. Just as an ineffective flash of the primer in the pan of the musket would result in no explosion of the charge, so a person who failed to live up to his early promise came to be known as a flash in the pan.
Received with thanks from Steve:
I've heard that 'flash in the pan' refers to pioneering photographers
who used magnesium powder, held in a metal pan to provide the first
flash photography.
Received with thanks from Frank:
Another possible origin is from prospecting when sun hit the pan and the
reflection was thought to be gold.
Received with thanks from Jeff Foraker:
A camera movement is called a pan. A actor/actress who gains attention
during a short film sequence can be referred to as a flash in the pan if the
actor/actress doesn't prove to have star quality.


NICK OF TIME
This goes back many centuries before the computer age, when accounts and scores were kept on notched sticks of wood, called "tallies." In a contest - soccer, for instance - the tally stick would be nicked each time one side scored. When a last minute score brought victory to one team, that nick was called the nick in time.
The wooden tallies, by the way, were a very important part of the official bookkeeping of the British Government for many centuries. Records were made of sums loaned to the government and the tallies were notched as each repayment was made. This practise was discontinued in 1826 and the frugal lawmakers decided to use the old wooden tallies to stoke the fires in the stoves in the House of Lords.
When this explanation appeared in a newspaper column, it brought an indignant complaint that soccer wasn't even invented until the nineteenth century, so how could ancients have been notching scores for a game that didn't exist? Well, it's a fact that the rules for "association football," as soccer is properly named, were not codified until 1863. But - and it's a mighty big but - these rules merely "brought uniformity to a sport that had existed in diverse forms for many centuries." The words are those of the Encyclopedia International - an authoritative source, even if a chap named William Morris was the executive editor of it. In fact, the encyclopedia notes that the game in various forms was known in ancient
Greece and in China, where they probably ran the score up on an abacus.


ALL GREEK TO ME
All Greek to me goes back to Shakespeare. The line was first spoken by Casca, one of the conspirators against Caesar in the first act of Julius Caesar. He was speaking of the comments made by Cicero after Caesar three times refused the crown of emperor. "It was Greek to me!" Cicero actually did speak in Greek, using that language as a device to make sure that casual passers-by did not understand his remarks. Today the expression "It's all Greek to me" simply means that what has been said is beyond the speakers's understanding.


APPLE OF ONE'S EYE
The first apple of the eye was the pupil, which in ancient days was thought to be a round object similar to the apple. As recently as Anglo-Saxon times, the same word, aeppel, meant both "eye" and "apple." It goes without saying that the pupil of one's eye is very precious indeed - and that's how the expression the apple of one's eye came to mean something greatly treasured.