When a large amount of "anus juice" is suddenly released from the anus, resulting in splash damage and a very strong stench being unleashed. Usually a side effect of taking a yakko, or large amounts of anal sex.
Bill: Ah shit man
Timmy: What is it ?
Bill: My ass, it feels all.....
Timmy: Fuck dude, you just wacked all over my new carpet
Bill: Wacked my new trunks too. Get me a tissue