Green’s Dictionary of Slang

(Aus./US campus) a ducking of someone’s head in a toilet bowl; also as v.

1987		Eble Campus Sl. Oct. 6: swirlie – stick someone’s head in the toilet.
1987		G.A. Fine With the Boys 173: The idea of a swirly – sticking a boy’s head in a toilet (either clean or unflushed) and flushing – was learned by some [...] youngsters attending a summer hockey camp.
2004		Beastie Boys ‘Oh Word?’ 🎵 Stick your head in the toilet, give yourself a swirly.
2017		T. Spicer Good Girl Stripped Bare 30: The school bully is named Madonna [...] she threatens to ‘swirly’ you in the dunny.