A time when people get together and drink a lot of alcohol. It is British, informal, and a little crude. A leaving piss-up is drinks after someone quits a job. A Friday piss-up is the pub after work, with the aim of getting drunk, not a quiet dinner. You can have one in a pub, a flat, or a park; the place matters less than the drinking. Though actually, it does not have to be huge or wild — a small group can still call the night a piss-up if the point is the beer. One famous joke says someone “couldn’t organise a piss-up in a brewery,” meaning they are hopeless at organising even the easiest party. Do not use this in formal writing. A milder way is drinks, a night out, or a drinking session.