A girl who is only attractive from three sides, with the fourth being the front, particularly because of a lack of breasts, or a highly unattractive face.
Guy 1: Dude, check out this chick's ass. I bet she's freaking gorgeous!
Guy 2: Be careful. She's a 270º girl.
(Girl turns around and smiles, revealing a toothless, dirty mouth.)
Guy 1: Awwww... dude. You were right. That's nasty.
Guy 2: I told you. You should have listened.