We got going on expressions that no longer refer to anything that we know about, such as "dead as a doornail," credited to William ("Piers Plowman") Langland, 1350. Doornails were long used to strengthen doors, being hammered all the way through the board then pounded flat, bent so it would be more secure, a process called “clenching.” This rendered the nail unusable for any other purpose. Thus, the bent nail was commonly called “dead."
Here's a whole bunch more, some of which I looked up, many of which I didn't:
- "Upset the apple cart" refers to the way apples are stacked neatly. So to upset the apple cart is to turn received wisdom on its head.
- Close-knit (we talking sweaters here?).
- Under the gun.
- See fit.
- Squared away.
- Go off half-cocked (my mother said this a lot).
- "A flash in the pan," meaning something that had its 15 minutes of fame, never to be heard from again, comes from the era of the flintlock gun, when the powder in a flintlock's pan could go off with a flash but not the main charge in the barrel.
- On track.
- "Ducts in a row." I assumed they mistaken wrote this rather than "ducks in a row," then wondered what that meant. In fact "ducts" is correct—this originates in mechanical engineering and refers to the importance of lining up air ducts if one expects the air conditioning and heating system to function properly.
- Caught red-handed.
- Close but no cigar.