1). A forceful wrist snapping action using your pinky and ring finger to hit somebody else typically in the forearm, triceps, head, ear, etc.
A common example of this is the ever popular 'dead finger' game that high school freshman and
sophomores use on each other in challenges of rivalry and
competition to see who can take the most pain before
giving up to the other in surrender.
2). The sound made by getting " thwapped" by someone else.
Scenario 1
sneak attack: boy1 is following boy2 in between classes, he sneaks up and thwaps his buddy in the exposed triceps while he chats up a girl he's walking with...
Scenario 2: boy1 and boy2 are friends and they thwap each other on a turn basis to see who's toughest.
Both leave the game with bruises. But there will be a winner.
"Bro I thwapped
the shit out of Brad -
that fucker is going to want pay-backs!" (Laughter)
"Thwap!" (Makes its own sound)