'assman' originates from English, specifically the compound of 'ass' (a vulgar term for the buttocks) and 'man' meaning 'adult male'.

'ass' (meaning 'buttocks') derives from Old English 'ærs' meaning 'buttock', which evolved into Modern English 'arse' and in some dialects/US usage became 'ass'. The compound 'assman' is a modern slang formation combining this element with 'man'.

Initially, the component 'ass' simply referred to the buttock; over time the compound 'assman' arose in informal speech to mean either a person attracted to buttocks or, jokingly, a specialist who deals with that part of the body (proctologist).