(1) Sour or bitter tasting; acid. (2) Sharp or biting, as in character or expression.

In a sentence (1):
Geez, I wish someone would have told me how acerbic the lime was before I ate it whole and then stuck it in my eye!

In a different sentence (2):
Although Professor Geraldine makes it seem that she’s accessible and friendly by allowing us to use her first name in class, during office hours her demeanor is totally acerbic.

As part of a familiar riddle turned horrific (and British):
What’s black, white, acerbic, and red all over?
Answer: A bloody unfriendly zebra!