'atavus' originates from Latin, specifically the word 'atavus', where 'avus' meant 'grandfather' (the exact role of any prefix is not clearly defined).

'atavus' was used in Classical and Medieval Latin and was later borrowed into English as a learned/antiquarian term (appearing in English usage from around the 17th–18th centuries) while generally retaining its Latin form 'atavus'.

Initially it meant 'grandfather' or a remote ancestor; over time in English it has come to be used chiefly for a 'remote ancestor' or forebear, often in scholarly or literary contexts.