'aussie' originates from Australian English, specifically as an informal shortening of the word 'Australian', where 'Australian' ultimately traces to New Latin 'Australia' and Latin 'australis' meaning 'southern'.

'aussie' changed from the longer form 'Australian' (used since naming of the colony/nation) into the colloquial shortened form 'Aussie' in 19th-century Australian English; 'Australian' itself came via New Latin 'Australia' from Latin 'australis'.

Initially it referred to something or someone from the land named for being 'southern'; over time it evolved into the casual, informal term 'Aussie' meaning 'an Australian' (used as both noun and adjective, often affectionate or colloquial).