The sound that an above average in size penis makes as it hits something. Generaly, another person or some inanimate object.
"Lets get ready to rummble baby"
*pulls out penis*
*thwapt* (hits leg, near the knee)
*mst mst mst mst*
*thwatp* (hits some surface, generaly bed)
*pat pat pat*
*cig, gets lit*