Green’s Dictionary of Slang

(Aus.) tight-fitting (usu. male) swimming briefs.

1935		Sydney Morn. Herald 19 Dec. 8/1: [advt.] Men’s Speedos in Wool [...] Women’s Speedos from 13/6.
[1953		Sun. Herald (Sydney) 13 Sept. 35/2: [advt] Youngsters [...] when it comes to swimshorts often prefer Speedo lastex while father may find Speedo’s boxer short more to his taste].
1990		T. Winton Human Torpedo 40: Lockie went wheeling back onto the sand [...] with a big toe in his ear and half his Speedos up his nose.
1999		G. Seal Lingo 25: The numerous terms for the ‘shorts’ worn when swimming, known variously as bathers (Western Australia), swimmers (New South Wales), cossies or costumes (New South Wales and Victoria), togs (Queensland) and speedos, or trunks in different parts of the country.
2011		J. Patrick (ed.) Smooth and Sassy 51: I felt my Speedos being peeled down so the waistband was tight under my balls.