A term used for somebody that is just sexually attracted to somebody. They just wnats to have sex with them, not date them or anything.
Guy 1: Look at that girl! She's hot, why dont you date her?
Guy 2: Nah, I'm just up onto her.
Girl 1: Wow, that guy stares at me! I wonder if he wants to date me?
Girl 2: I think he is just up onto you.
19 Sentyabr 2009