When having sex doggy style, when the guy is about to ejaculate, he pulls out and then proceeds to spit on the girls back so that she turns around, and blows it right in her eye.
Girl: You're done already?
Guy: Sure am...
(Girl Looks around)
To firmly slap ones penis across another persons face i.e. to be either very manly or very homosexual.
"Dude! That little limp wristed faggot just thwacked me!"
One jock buddy says to another...
"WHOA! Billy's asleep, someone thwack him!"