The headword is the present middle/passive imperative, second person singular, of the verb megali/zomai, I bear myself proudly; see generally LSJ s.v. It is quoted from Homer, Iliad 10.69 (web address 1), with scholion: see next note. There, Agamemnon (cf. alpha 140 and OCD(4) s.v.) sends Menelaus (OCD(4) s.v. Menelaus(1)) to rouse other Greek leaders for a night raid on the Trojan camps and advises him not to be too proud to carry out the chore (Cunliffe s.v., p. 258).
[1] The gloss is the same form as the lemma, but from the verb e)pai/rw, I exalt, lift up and set on; see LSJ s.v. and epsilon 2014. The headword is identically glossed at ps.-Zonaras 1350.13 and (so Adler) Ambrosian Lexicon 527. It is similarly glossed in the scholia to Homer, Iliad 10.69.