(plurale tantum, UK, Irish, Commonwealth, informal) Briefs that have a fly as part of an inverted Y formed by the seams.
I did the latter, so in writing this I have put it on record that I am one of the few men who has spent Christmas Eve running across a Kentish marsh in his Y-fronts with two horses trotting after him in close company.
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Regional slang from around the English-speaking world — British, Australian, Irish, Caribbean, Nigerian, Filipino, AAVE, and the hyphenated-English dialects that make the internet sound local.
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