expression, noun-modifying (irregular)
Vulgar idiomatic expression literally meaning 'one's asshole is small', used to describe someone as narrow-minded, petty, or lacking courage. Strongly colloquial and offensive.
あいつはケツの穴が小さいから、ちょっとしたことで怒る。
That guy is so petty he gets angry over every little thing.
ケツの穴が小さい上司のせいで、新しい提案が通らない。
Because of our small-minded boss, new proposals never get approved.
A vulgar idiom from the image of a small anus implying a constricted, ungenerous personality. The exact origin is uncertain, but it is a crude metaphor for pettiness and cowardice.