I'd better make a quick disclaimer on this one: sometimes there are many theories as to how a phrase came about. These 12 explanations are just some of the possible origins. We've been using some of these phrases for so long that we've lost the original meanings, so our explanation of them is based on the the best guesses of linguists and historians. Take the explanations with a grain of salt (haha).
1. "Not worth his salt." In Roman times, salt was a highly valued commodity used for trading. To say a soldier was not worth his salt was the same as saying he wasn't worth his salary; he was absolutely worthless. Photo from What's Cooking America.
2. "Pie in the sky" is actually only half of the phrase - the whole thing is "there'll be pie in the sky when you die," and it's a sarcastic remark that means heaven is a silly notion.
3. Money is sometimes called "dough" or "bread" because money is what puts the bread on the table. By that logic, the two are basically interchangeable.
4. "Egg on your face" may come from the times of Victorian live theater. While we're most familiar with the fall guy getting a pie in his face, Victorian theater had the embarrassed party getting raw eggs cracked over his head. However, another explanation suggests that people who eat eggs often get yolk all over their faces, which is embarrassing. Photo from DippingEgg.com.
5. "Won't amount to a hill of beans" (or the like) comes from the practice of planting bean seeds in clumps in a mound of soil (the hill). This is a very small hill indeed, so saying you won't amount to a hill of beans is pretty insulting.
6. "Apple of my eye" is thought to have originated from an old English idea that the pupil of the eye was solid, like an apple. So the "apple of my eye" is the pupil of my eye. I guess that sort of poetically means what catches my attention most.
7. "Cool as a cucumber" exists because the high water content of a cucumber keeps them pretty cold. Lettuce and celery both have high water contents as well, but I guess "cool as lettuce" doesn't have the same ring to it. Photo from FoodMomiac.
8. "Cream of the crop" is because if you have a pail of freshly-squeezed milk, the cream will rise to the top of the pail because of the high fat content. Since cream is so rich and delicious, it's considered the best - so if you're the cream of the crop, you're obviously the best!
9. "Top banana" and "Second banana" probably come from the same place. The term comes from the early 1900s vaudeville days, and may have come from comedian Frank Lebowitz, who used bananas in his act.
10. "The greatest thing since sliced bread" is pretty self-explanatory - how great is it to just pull out a couple of pieces of bread and not have to be bothered with getting out a knife and trying to cut even slices without hacking up the loaf? It's hard to believe, but pre-sliced bread actually wasn't really a practice until 1928 and wasn't marketed until 1930 by Wonder Bread.
11. "Cut the mustard" has always seemed pretty strange to me, but it actually makes sense: it means to be up to a challenge. And if you think about it, cutting mustard? Pretty difficult. Photo from English Shop.
12. "Dollars to doughnuts" means "most assuredly," which I explain because I'm not sure how common it is. I use it, but I don't know if it's weird midwest slang or what. An example would be, "Dollars to doughnuts, Heath Ledger is going to win the Best Supporting Actor Oscar." Anyway, it comes from the fact that if you're willing to bet dollars to something that's essentially worthless (although Homer Simpson would probably argue with you), you must be pretty sure that you're right. Variations include dollars to buttons, cobwebs and dumplings.