That kind of talk is too coarse — don't bring up 'asshole'.
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Crude. 屁 (pì) means 'fart' and 眼 (yǎn) literally 'eye/hole', so 屁眼 is roughly 'fart-hole'. Used in insults like 没屁眼 ('lacking an asshole', i.e. heartless / worthy of being cursed). Avoid in polite speech — clinical term is 肛门 (gāngmén).
