Green’s Dictionary of Slang

(Aus.) male underpants.

1966		Baker Aus. Lang. (2 edn) 259: Underdaks and underchunders, underpants.
1977		Aus. Women’s Wkly 7 Sept. 171/3: [advt] Holeproof Cotton Underdaks pack of 3.
1997		Bug (Aus.) 9:5 18 Dec. 🌐 And as he moved fluently to the bowling crease, I provocatively discarded my gloves, protector and pads, unzipped the whites and plopped the plums up and out of the underdaks so they looked like a startled bull frog peering over a log.
2002		Dimmey’s Online (Aus.) 🌐 Holeproof Cotton Underdaks. The original Holeproof Underdaks Cotton Hipster. Available in sizes 75–110, and in blue, red, black or lime.