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(Australian): Underpants.

Derives from Sydney Rhyming slang, "Reg Grundies = Undies", which is a reference to that excellent Australian TV production outfit Reg Grundy, behind such televisual classics as Neighbours, Home and Away, The Sullivans, etc.
From "The 12th Man Again", wherein commentator Max "Tangles" Walker discusses the perils of prolonged bouts of commentating on international cricket competitions:

"when you're sitting around in the commentary box for this length of time, it's not unusual, really, for the old cods to maybe nod off, get a bit numb; I think the best thing to do in situations like this is to get up, stretch the old legs; maybe even try to casually slip the old hand down and, maybe, give them a bit of a rub; probably try and adjust the old reg grundies while you're down there; try and loosen them up a bit. Anything, really, to try to get the old circulation going again."
by Blind Electric Ray May 03, 2005
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We gave the stupid little freshman a wedgie after he asked us where the elevator was.
by Popsicle May 18, 2004
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Going missing in a brothel for a week, smashing loads of bag before going home and pissing and/or spitting on your family
I fucked up this weekend, done a grundy didn't I
by Reegzie December 14, 2018
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Reference to one's huge ego. Typically paired with English/Scottish males named Joe.
"Wow! He seems a bit far up himself!"
"Yeah he is a bit of a Grundy."
by Twollamas May 16, 2017
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verb: to sneak up behind someone, grab their underpants, and yank them forcefully upwards causing their underpants to get jammed up way deep in their butt crack.
dude, did you see what happened to that new fat kid on the playground? He got a grundy so hard it ripped the elastic off his nuthuggers!!!
by shellacked June 17, 2008
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When that fat chick spread her legs, that shit looked straight grundy!
by StillBorn May 01, 2003
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