Y-fronts - Wiktionary, the free dictionary

Y-fronts pl (plural only)

1. (plural only, UK, Ireland, Commonwealth, informal) Men's or boys' briefs that have a fly as part of an inverted Y formed by the seams.
   • 2013, Mike Peyton, Mike Peyton’s Floating Assets, page 4:

     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.

• See Thesaurus:underwear

• jocks
• bibidees
• bikini briefs