The headword, in epic typically an epithet of goddesses or high-born women, is a two-ending adjective in the masculine and feminine nominative singular; see generally LSJ s.v. and Cunliffe, p. 373. The entry is generated from instances in Homer, e.g. Iliad 3.228 (on Helen: web address 1); cf. the scholia there, and Apollonius' Homeric Lexicon 149.8 (whose gloss on the headword this entry evidently follows).
For compounds with tanu-, see also tau 83, tau 84, tau 85, tau 87, and tau 89.
[1] Identical glossing in the Synagoge (tau29), Photius' Lexicon (tau45 Theodoridis), and Etymologicum Magnum 745.45 (Kallierges); cf. also e.g. Hesychius tau148. [In her critical apparatus Adler reports that ms V lacks the additional clause e)c ou(= ... dhloi=.]