When having intercourse, as the action heightens, the splack is the sound that the two bodies make as they come together with excessive force. Not to be confused with "smacking that ass," splacking is a sound of body parts "smacking" together.
Also, they act of really "hitting it hard" or "putting it on her" as they often say, which amounts to having some really furious sex.
"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. "It only took me two minutes to make that bitch's pussy splack."
"By being Splack, I am a serious threat to the chicken population."