In principle, it would seem to come from the Latin medianos “median lands or territories”, “situated between two places”, although the name also found in Galician – although it is not the popular one there – is As Maus, and for this toponym it has been suggested that it comes from Asmanos as a gentilicio, in reference to people from the valley of the river Asma. The early and erroneous deglutination of the initial as- interpreted as an article would have led to this result. See Domínguez Quiroga (171) and Boullón Agrelo (16).

Cited bibliography: 

-Boullón Agrelo, Ana Isabel. “O artigo na toponimia galega: aspectos da estandarización.” Revista Galega de Filoloxía 12 (2011): 11-35.

-Domínguez Quiroga, Manuel. Toponimia da provincia de Ourense. Ourense: Deputación Provincial de Ourense, 2017.