v. or n.-depending on usage.
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.
All I know is that this girl is crazy and tonight I'm gonna splack that ass!!