It happens in the middle of the night when you wake up with a massive dookie pain, rush to the bathroom and don't notice the toilet seat is up. A bare butt hitting cold water at 2am usually causes the person to yelp loud enough to wake everybody in the house.
Julie: "Eeeee-ahhhh!"
Kelly: "What happened in there?"
Julie: "You left the freakin' toilet seat up again! I had a splashdown!"
Kelly: "I bet you're awake now."
"Spladow" is the onomatopoetic exclamation that a gentleman shouts while delivering a handfull of of ejaculate to the face of a partner via throwing in a sidearm motion.
Brought into popular use by Godfrey, a comic, on a VH1 Episode of "I love the 90's"