Sharing spit is slang for making out. It literally describes the act of making out, because usually when two people are kissing with tongue spit is exchanged. It could be used in a negative or positive way depending on who you're talking about (but usually used negatively, e.g. when talking about a conniving weasel of a girl cheating on her faithful boyfriend.)
Mackenzie: Hey, Shawna, did you see Khloe sharing spit with that white boy at the game?
Shawna: Yeah, I saw it. And her boyfriend was like right there. What a whore.
Mackenzie: I hate Khloe.
Shawna: Yeah, I saw it. And her boyfriend was like right there. What a whore.
Mackenzie: I hate Khloe.
by manda15 December 3, 2013