The headword is third person singular, present indicative active, of the verb margai/nw.
Same entry in other lexica (see the references at Photius mu107 Theodoridis), beginning with Hesychius mu260, and cf. also the scholia to Homer, Iliad 5.882, where the infinitive margai/nein occurs and is glossed with mai/nesqai.
The only literary instance of the word as cited (which is perhaps unlikely to underlie Hesychius) is Gregory of Nazianzus, Poems about himself (Carmina de se ipso) PG 37.1315.12.
For related words see mu 184, mu 186.