Growing up, my folks always encouraged us to learn by looking up any word or phrase we didn't understand. There was always a nearby dictionary, bible, and encyclopedia at the Kellett house, ready to solve arguments over word usage or phraseology.
As I've aged, that's an impulse that's served me well. But it's transmuted into a weird interest in word and phrase origins.
Today's use of "dead as a doornail" got me wondering how old that phrase is. Turns out, it's from the 14th Century! If you're interested, here's a good description exploring the possible origins of the phrase.
(Want another good one? Look up the origin of the phrase "saved by the bell" from the Black Plague days. That'll leave you terrified for a while.)