also: しりのあな
expression, noun
Vulgar term for the anus. Used in crude or insulting contexts. The reading けつのあな is the most common; しりのあな is less frequent and may sound slightly less harsh.
あいつは本当にケツの穴の小さい奴だ。
That guy is really a tight-ass.
そんなこと言うと、ケツの穴をほじくるぞ。
If you say that, I'll poke your asshole.
肛門 is the formal medical term for 'anus', while ケツの穴 is a vulgar slang expression.
尻 means 'buttocks' in general, whereas ケツの穴 specifically refers to the anal opening.
Compound of ケツ (slang for 'buttocks', from 尻) and 穴 ('hole'). The reading けつ is a vulgar variant of しり.