Foolscap Paper
If you grew up in the British Commonwealth, you’ve probably heard of foolscap paper. It’s a slimmer size than A4, and a bit taller (8 x 13 inches, usually). Most of our bigger notebooks in school were foolscap-sized.
Except, when we were in school, we pronounced the name “full-scape” (assuming it had the same origin as landscape etc.) and thought it was “full”-sized, as opposed to the half-sized notebooks we did our homework in.
I read the correct spelling at some point, went “oh”, and updated my spelling without wondering the word might actually mean, until a few days ago, when I read “foolscap paper” in something and thought, wait, does this have anything to do with a fool’s cap?
Lo and behold, turns out it was a folio size called that because it had a watermark of … a fool’s cap and bells.

The name became a generic applied to that size of paper. Not quite genericide, but close.
For bonus points, here’s the Fool’s Cap Map of the World, which is one of the eerier maps you’ll see.
