The original meaning of πρύτανις was 'chief magistrate' which metonymically could also be used of the ‘ruler, lord’ (e.g. Zeus). It is no doubt rightly connected with the Etruscan magistrate's name purθne, eprθni. Because of this, and because Aeolic has πρότανις, Phocian and Cretan βρυτ-, it cannot be a native word but must be a borrowing. Hittite hupurtanuri- is an official's title, known from Ugarit. Laroche has interpreted it and the words tuppalanuri-, tuppanuri- as containing the adjective uri- ‘great’, so that tuppalan-uri = ‘grand des scribes’, tuppan-uri = ‘grand des tablettes’; this would leave us with a basic *huburtan- which seems to have been taken over as οπορταν-/οπροταν- or οπυρταν-/οπρυταν-, and by deglutination of the ‘article’ gave πρότανις/πρύτανις.
Source: wiktionary