I was going to say "mango" (I don't mind underripe avocado; most of them are like that here), but then I remembered what underripe red bananas are like, and OMGBLAARGH. It's like eating highly astringent chalk that sucks all the moisture out of your mouth, and it tastes vaguely cucumberlike. I genuinely thought for a minute I'd eaten something poisonous, but then I checked out the internet and nope, that's just what they're like before they ripen.