A gathering of people who meet up before they go on a night out to drink as much alcohol as possible.
This is usually done by students to save money. It also adds a bit more to the night and there can even be an after-swall on special occasions.
Chrissy - yo mate you going out tonight?
Ben - yeah i think so are having a pre-swall?
Chrissy - yeah pre-swall at mine at 8 oclock.
Ben - Okay, see you then, let's get fucked!