A person who is really mean, annoying, or hussy-like. If you are frustrated or mad at someone you can call them this without cussing! It's basically a b****, meanie, jerk, etc.
Brooks: Oh my gosh, she was really mean to him.
Catherine: I know, she is such a knix.
Rae: Look at that knix!
Robert: Yeah, that skirt is so short, jeez!