'asshead' originates from English, specifically the compound of 'ass' and 'head', where 'ass' originally referred to the animal 'donkey' and later came to be used figuratively for a 'fool' or 'stupid person', and 'head' meant the physical 'head' or a person (as in '-head' forming nouns).

'ass' comes from Old English 'assa' (ultimately from Latin 'asinus' via Germanic), and 'head' comes from Old English 'heafod'. The compound 'asshead' is a relatively recent colloquial formation in modern English (attested in informal US/UK speech in the 20th century) created by combining these elements as an insult.

Initially the separate words referred to an animal ('ass') and a body part/person ('head'); over time the compound evolved into a slang insult meaning 'a stupid or obnoxious person.'