Vocab fun—no it's not an oxymoron

So, until recently, I had been studying for the GREs. And at least for the verbal section, it was a lot of looking up specific definitions for words of which I had a general sense of the meaning—acrimonious, lascivious, hegemony, moribund ... fun ones like that.

But along the way, I was surprised to find that I was totally off-base on the definition of callow. In my head, I would have thought sarcastic, biting, acidic, or something like that. But it means "lacking adult sophistication: immature." A very useful word when you know what it means.