The headword, presumably extracted from the quotation given, is the neuter nominative/accusative singular form of the adjective megalei=os, -a, -on; see LSJ s.v., mu 357, mu 358, mu 359, mu 361, and epsilon 1348 (end).
[1] The gloss is the neuter nominative/accusative singular of the adjective megalopreph/s, -e/s; see LSJ s.v., and the masculine and feminine nominative singular at mu 369.
[2] The Carthaginian general Mago(n) (243-203 BCE; OCD(4) s.v. Mago(2), and mu 27), whose name appears here in the dative, was the youngest son of Hamilcar Barca (OCD(4) s.v. Hamilcar(2), and alpha 1596) and brother of Hannibal (OCD(4) s.v., and alpha 2452) and Hasdrubal (OCD(4) s.v. Hasdrubal(2)). After Mago's surprise attack and capture of the port town of Genua (present-day Italy's Genoa, in coastal Liguria; Barrington Atlas map 39 grid D5) in 205, during the Second Punic War (218-201), he effected an alliance with the resident Ingauni tribe (Smith, vol. II, 53-4) and recruited among the Gauls (Briscoe, 56; Livy 28.46). But despite having thus substantially bolstered his army, in 203 Mago's troops were routed and the general gravely wounded in a major battle with a Roman force four legions strong near the Insubrian (OCD(4) s.v. Insubres; Smith, vol. I, 936) capital of Mediolanum (present-day Milan, Italy; Barrington Atlas map 39 grid E3); cf. Livy 30.18.
[3] Polybius fr. 180 Büttner-Wobst. Büttner-Wobst notes (538) that this fragment was attributed to Polybius by Valesius (Henri de Valois 1603-1676). The fragment's place is uncertain. Schweighäuser associated it with Livy 30.18 and events in Insubria of 203, so that it should be located within Polybius' book 14 (Büttner-Wobst, 538). Meltzer, however, gesturing at Livy 28.46.8 ff., placed it the context of Mago's intervention in Liguria (ibid.), from 205-203; the source would thus be a lost res Italiae of Polybius book 13 (Walbank, 754).
[In her critical apparatus Adler reports that mss GM read to\n *ligusti=non (the Ligurian, in the accusative singular): "those carrying on the war with Mago against the Ligurian."]