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!
by millheadmate May 12, 2010
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