To dump salt upon, usually unnoticed. While sitting at a restaurant, one 'salts' a person next to them by grabbing the salt shaker and quietly pouring salt on their lap until the victim notices. When the victim discovers that they have been salted, the one who salted them usually shouts "HAHA! SALTED!"
John: Sally, what are you...?
Sally: Shhh! I'm salting Susan. *pouring salt on Susan's lap*
Sally: HAHA! SALTED!