A rude, nasty, verbally aggressive person (usually a woman but not always) who makes snide comments through a fake, neocortex smile.
Normal Person: I talked to someone at the baggage claim who said that they would have my bag ready within four hours.
Smiling Bitch: Oh. Well, I guess that they must have been wrong.
Normal Personal: Why are you smiling?
Smiling bitch: Because I'm not just a bitch, hon, I'm a smiling bitch.