Afrikaans in origin - poes (vagina) and klap (slap). Pronounced "poosklup" with long "oo". Rhymes with goosecup if that was actually a word. It is the type of slap administered to a recipients head in such a way that their eyes bug out, their sphincters let go and their descendants end up sporting black eyes and stunted mental development.
Juslike! Piet got a poesklap from that dude! He didn't check it coming and it sent him spinning like a top. Piet shat his pants and dropped like a rock. Aira! That poesklap was a real tit shot to the jaw. Like Rocky Balboa.