Where does “Avestic” come from?

Avestic (English) comes from English Avesta, from Persian اوستا, from Middle Persian ʾp(y)stʾk'.

Ancestry of “Avestic”, step by step

Avestic traces back through two separate lines of ancestry.

via English Avesta

StepLanguageWordMeaning
1EnglishAvestaAvestan language
2PersianاوستاAlternative form of استاد
3Middle Persianʾp(y)stʾk'

via English ic

StepLanguageWordMeaning
1Englishicrelated or pertaining to; having the character of
Every word from Middle Persian ʾp(y)stʾk'Every word from Persian اوستاEvery word from English Avesta
Avestic — Etymology, Origin & Word History | Etymology Explorer