grundies, n. — Green’s Dictionary of Slang

grundies n.

also gruds, grunds

[rhy. sl. = undies n.; ult. Aus. media executive Reg Grundy (b.1923) (cf. Reg Grundys n.); UK use is more likely to be based on Eddie Grundys n.]

(N.Z./Aus./UK) underwear.

1988		Bulletin issues 5626-33 105/2: Reg Grundies: Items of intimate apparel.
1988		McGill Dict. of Kiwi Sl. 53/2: grunds/grundies underwear; rhyming slang Reg Grundy/undie.
2001		OnLine Dict. of Playground Sl. 🌐 grundies n. dirty underwear.
2006		C. Marriner Southern Style 10: I [...] stormed from the bedroom with me grundies halfway on.
2011		Times Mag. 30 Apr. 14/1: Maybe you call them undies, knickers, briefs, gruds, Alan Whickers?