More commonly used in Australia; basically a social gathering or full-blown party intentionally to get drunk. Commonly associated with pisswrecked, pissed, wreckless, drunken, wrecked, fucked.
In the holidays, there was a massive-arse piss-up to celebrate no school. 'Twas well-sick :)
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.
Some random: "The other night was a prooty well-sick piss-up."
Me: "Yeah I got well pisswrecked!"