noun
O-back; skirt with a peek-a-boo hole in the rump
A fashion item, typically a skirt or dress, with a cut-out or opening at the back that reveals the buttocks. The term is a wasei-eigo coinage from 'O' (shape) + 'back'.
彼女はパーティーで大胆なＯバックのスカートをはいていた。
She wore a daring O-back skirt at the party.