The usually odourless fluid/mucus that leaks out of your ass after pro-longed anal stimulation. It isn't semen, but rather your own body's colon lubrication that has pooled up behind your anus expecting more rectal stimulation. Usually comes out in one large blob like a loogy. Beware of farting while this is inside of you.
Joan - "Boy, after Max reamed me for hours last night, I've done nothing but fart anal fluid."
Sally - "I know what you mean. For hours after a really long anal session with Joe I feel like I'm foaming at the ass!"