From Proto-Balto-Slavic *awis, from Proto-Indo-European *h₃éwis.
avs f (6th declension)
- (dated) ewe (female sheep; also generic term)
avju gans ― shepherd
avs piens ― ewe(= sheep) milk
The term aita is more frequent than avs, which is considered dated, both as a generic and as the specific name of the female.
Declension of avs (6th declension)