The headword is a feminine noun in the nominative (and vocative) singular; see generally LSJ s.v. pre/sbus, the masculine equivalent. Its earliest extant attestation is in the Homeric Hymn (5) to Aphrodite 32 (web address 1); thereafter in tragedy, comedy, and other poetic genres.
[1] The gloss is a two-ending adjective in the masculine and feminine nominative singular; see generally LSJ s.v. The headword is similarly glossed (but with the superlative) by ps.-Zonaras 1577.1; cf. Hesychius s.v.