noun
buttock cleavage exposure; visible butt crack
Slang term for when the top of the buttocks is visible, typically because pants are riding low or when bending over.
あの人、座ったら半ケツになってるよ。
That person's butt crack is showing when they sit down.
ズボンがずり落ちて半ケツが見えてる。
His pants are slipping down and his butt crack is visible.
Compound of 半 (half) and ケツ (slang for buttocks). The exact origin is uncertain, but it likely emerged from casual youth slang.