For years, I've kept a Words to Learn list, to which this thread has added. I write the word, its part of speech, and its definition. The list includes words I mix up (callus, callous) and words with subtle differences in meaning (coffin, casket).
When I finally know a word on that list without having to think, I move it to the end of the list, where there's a too-short section Words Learned. It's almost embarrassing how simple some of those are, and how wide their use, before I finally got 'em down.
Allow me to contribute from my list:
casket, n.
A burial box that is rectangular and does not narrow at the head or feet. First in use in the US as late as 1849. cf. coffin.
coffin, n.
A burial box that narrows for the head and feet, like the Halloween decorations do. In use long before casket.
Maryn, not quite too old to learn