The little amount of beer left in bottles, cans, cups, etc. that nobody ever wants to finish. Spit Slosh is warm, flat, and the main reason for leaky garbage bags at parties. It can most commonly be found in beer pong cups after a sloppy game.
Girl: "Hey can I get some of your beer?"
Guy: "Sure you can just finish it I was about to grab a new one anyway."
Girl: "Nevermind I'll just get my own, I don't want your Spit Slosh."