To take a shit and masterbate at the same time.
"hey, I'm bored, and I've got to take a shit. I guess I could just go shwank and kill two birds with one stone"
A knacker in Fingus trying to be cool
When a knacker goes up to another knacker, and to act like hard ass hey will scream "SCHWAA", like any animal in the wild trying to act scary, or fend off threats