Juice from tits that has become thick, creamy, and rancid more like rotten butter than milk for some reason. Possibly from an old ass lady or someone that has died.
Rick: I was banging that pregnant hooker last night and she sure as hell as some sour milk tits.
Jake: Maybe she has been eating a lot of those sour Warhead motherfuckers
Rick: I don't know, sucks balls to be that baby