The authors agree that this surname derives from the Slovene term mežnar (also written meznar), which means "sacristan", and which in turn comes from the German Mesner, taken from the Latin mansionarius. According to the lexicographical tradition cited by B. Zuanella, the mansionarius was entrusted with the custody of the church and resided at the place of his service; in time, that function passed from priests to laymen. The surname would thus reflect the occupation of the founding ancestor.

Zuanella documents the presence of the surname at Antro —a locality of the comune of Pulfero, in the Natisone valleys— from the middle of the 17th century, although it soon died out in that area. Its memory survived in the house name Mežnarji, the traditional designation of the families where the village sacristan once lived. The derived forms Mežnarič and Mežnarčič are found diffused in various parts of Slovenia. Zuanella further notes that the Slovene dialect of the Natisone valleys incorporated numerous religious terms of German origin, among which mežnar belongs.

E. Rossoni indicates that the form Mesnar appears to have disappeared in Italy, while the variant Meznar, of Slovene origins, is recorded almost in isolation at Trieste.