Proverbially, the thing that a particular bad organiser couldn't organise, even if by pure dumb luck he happened to find himself in the place where beer is produced.
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 :)