intentionally degrading somebody.
got its name because of the stinging feeling one gets if somebody where to put salt on an open wound. the effects of "throwing salt" at a person are usually the same, but the stinging is on an emotional level.
Guy 1: "You guys are so weak. Yeah you won the game, but only because the other team blows
Guy 2: "Man, why you always throwing salt?"
Talking about things in a group, which at least one person within the group shouldn't hear/know.
Rick, Chris, Rob standing somewhere, talking:
Rick to Chris: Do you know, that Isaiah will move to amsterdam?
Chris to Rick: Oh man you are throwing salt all over the place. Rob shouldn't hear that.