Old Persian 
Generally assumed to be a loanword from Median (only found as a title? of persons that are also identified as Mede). Cognate of Avestan (moġu-) but probably not of Av. maga- (cognate with Skt. magha-, "gift"). Crucially, Middle Persian moabad does not descend from Old Persian/Median magu-.
Proper noun 
- unknown meaning; hapax legomena known only from the Behistun inscription where it is used as a title (?) of persons. Sometimes said to be the term for a Zoroastrian priest, or for a magician, or for a sub-tribe of the Medes; these interpretations are formed by Greek Μάγος and μάγος.
Taking Old Persian as representative for all Old Iranian: