A smilie used when someone is laughing at something they probably shoulde'nt, such as sexual enuendo or an immature or crude joke. it can also mean a cheeky grin. similar to :3 but looks less gay if a guy uses it in conversation, and is more of a responce.
Jill: I just swollowed a banana whole!
Jack: cool :}
Jack: I think i need a new ass
Jill: Why? :P
Jack: Mine has a crack in it :}