“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.
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.
“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.
“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.
More ‘food phrases’ explained here.