Etymology tree Proto-Indo-European *wed- Proto-Indo-European *-r̥ Proto-Indo-European *wódr̥ Proto-Hellenic *údōr Ancient Greek ῡ̆̔́δωρ (hū̆́dōr) Ancient Greek ῠ̔δρο- (hŭdro-)der. English hydro- English geology English hydrogeology From hydro- + geology.