Y-fronted

adjective2

1. •
   (UK) Having a Y-shaped seam, as Y-fronts do.

   • Underneath he was wearing white Y-fronted underpants. — 1975
2. •
   (UK) Wearing Y-fronts.

   • Squeezing open the kitchen door, Amy came face to face with Greg's baggy Y-fronted backside. 'Greg! Amy's here!' shouted Sophie, too late. — 2007