The sound which is made when a very wet substance hits a durable surface at force. This sound would be most commonly heard when an extremely wet turd departs from your anus and hits the surface of the water at speed.
Bill: "I had an extremely wet turd this morning"
Henry: "Did it make a Schlop?"
Bill: "Yes, my arse is still moist"
When one makes a puddle of spit in a cupped hand and proceeds to forcefully cup the balls into the pool of spit so hard that the spit splashes everywhere.