Australian slang; defines heavy intoxication through alcohol consumption. Other descriptions could be wreckless, pissed, drunken, wrecked, smashed, fucked. Find someone pisswrecked at a piss-up. Typically characterised by staggering around, slurred words...etc.
To slap a stream of urine back and forth with one's flat-palmed hand. Can be a solo or group activity performed by either gender. Generally practised at urinals and public toilets.
"My game was on point that night, I pisswhacked at a rate of 329wpm (whacks per minute)"
"I pisswhacked the shit outta this guy's stream at the urinal last night, his expression was one of liberation"
"Some guy started pisswhacking me at the urinal, the experience has brought me closer to Jesus"