I read a lot as a child. I talked a lot too, and I think early on my parents got me to "figure it out from context" when I'd encounter a new word.
They must have done that as well, as I was taught by their usage that to "lie" meant to say something that was not true. e.g. -- I lied, I'll take the coke not the iced tea.
In some cases, my guess was close, and I was in my 30s when I met an inconsistency and looked up ambivalent. I thought it meant to simply not care between 2 or more choices.
Yesterday (I'm no longer pulling 40, but rather pushing 50) I encountered "erstwhile" in a *blink* *blink* context. My childhood reading of that had indicated that it meant ardent, fervent, tenacious. e.g. -- Meanwhile, her erstwhile lover braved snowy mountain passes and hungry dragons in order to rescue the princess. *ahem* Not quite.