In a basketball game, when a male player jumps up to shoot the ball and another player is waiting for the rebound, the guy shooting might rub his balls in the other man's face/mouth. This is similar to sucking the salt off of a peanut's shell, hence, salties.
John: SLAM DUNK!
Aaron: Yeah, but I feel bad for the other guy.
John: Why man?
Aaron: Did you see that?! He got some salties.
John: Sick dude.