Definitions by The Pro One

GNO 

G- Get
N- Nob
O- Out

Thus literally meaning get your nob out
Usually used as an insult or as an obvious sex phrase

Boy: You're a loner and you've got no friends (Insulting)
Boy2:GNO

Or

Boy: Wanna have sex?
Girl: GNO!

GNO by The Pro One March 11, 2009