Where does “Авасто” come from?

Авасто (Tajik) comes from Persian اوستا, from Middle Persian ʾp(y)stʾk'.

Ancestry of “Авасто”, step by step

StepLanguageWordMeaning
1PersianاوستاAlternative form of استاد
2Middle Persianʾp(y)stʾk'
Every word from Middle Persian ʾp(y)stʾk'Every word from Persian اوستا