There are words I've been using my entire life and then, in one horrifying moment -- usually during a presentation to senior management, or when I accepted my Nobel Peace Prize -- will discover I've completely misunderstood their meaning.
Exacerbate is one of them. I always thought it meant to improve a situation (as in, "Those three scotch and sodas I had before dinner really exacerbated my mood.") but only recently did I learn it means the exact opposite, to make a situation worse (as in, "Those three scotch and sodas I had before dinner really exacerbated my mood.").
Mitigate is another. I thought offering up a defense of "mitigating" circumstances meant I should get off scot-free, whereas it actually means I'm still just as guilty of whatever I was accused of but might pay a fine instead of serving time in the slammer. I also learned as recently as typing that last sentence that it's not "scott-free."
Then of course there are the Toxic Twins of flammable and inflammable, which sound like opposites but mean precisely the same thing: burn, baby, burn. In kindergarten we learned to "Stop, Drop and Roll" if we ever caught on fire, and the day we became familiar with that protocol I ran home after school and asked my mother if the Batman PJs she'd just bought me were inflammable. "Of course they are, dear," my mother replied -- "I insisted on it." We had a difficult relationship.
I used to use nonplussed to express how something didn't faze me, that I took in stride. When I found out it means exactly the opposite I can describe my reaction only as... nonplussed.
And I've never understood how, if capitulate means to surrender, why recapitulate wouldn't mean to surrender again. Let me know if I need to repeat that last observation.
And it's not just the meaning of various words; sometimes there is a relationship between objects that surprises me. I don't think I knew that capers were flower buds until I was in my thirties. Or that cilantro and coriander are from the same plant. Or that Donald Trump has a sister who is a respected federal judge.
There -- I'm nonplussed again. Where's the scotch?