The only free and comprehensive online etymological dictionary of the Spanish language
maestro m. (Noun) "teacher"
10th cent. From Latin magistrum, accusative of magister 'id.,' from magis "great" and *-tero- (see más and -tr- respectively).
Unusual variants maestre, maese derive from the nominative and vocative cases of magister respectively. (Note that surviving reflexes from the vocative case are vanishingly rare in Spanish.) Also the origins of the surnames Maestro, Maestra, Maeso, Maese, Maesso, Maes, Maestud, and Mastro.
Asturian maestru, Portuguese maestro, Galician mestre, Catalan mestre, French maestro, Italian maestro, Romanian maestru, Sardinian maistru