'avestan' originates from New Latin, specifically the word 'Avestanus', where 'Avest-' referred to 'Avesta' (the Zoroastrian sacred texts) and '-anus' was an adjectival suffix.

'avestan' changed from New Latin 'Avestanus' into Middle English/Modern English forms like 'Avestan' and eventually became the modern English word 'avestan'. The name 'Avesta' itself comes from older Iranian sources via Avestan and Middle Persian.

Initially, the term primarily meant 'pertaining to the Avesta' (the scriptures); over time it came to be used specifically for 'the language of the Avesta' as a named historical language.