First known mention: The 2Live Crew song "Put Her in the Buck" in the following context:
"Lay the bitch on the bed, flat on her back
Hold her legs up high, make the pussy splak"
From this usage, it can be inferred that the word "splack," means:
1. (n) The noise that the penis makes when repeatedly "pounding" the vagina makes when especially lubricated. An onomatopoeia.
2. (v) The sound used as a verb, similar to "clap."
1. "Her pussy went splack, splack, splack."
1. "It only took me two minutes to make that bitch's pussy splack."