The headword megalh/twr is a masculine and feminine noun in the nominative singular; see generally LSJ s.v. A common epic epithet of heroes (Cunliffe s.v., p. 256), its appearance here is generated by Homer, Iliad 2.547, where it occurs in the genitive singular megalh/toros (web address 1), glossed in the scholia there by the same adjective used here: see next note.
[1] The gloss is a two-ending adjective in the masculine and feminine nominative singular; see LSJ s.v., mu 371 gloss, and a comparative at mu 372. Same or similar glossing in other lexica: see the references at Photius mu165 Theodoridis.