The headword, presumably extracted from the quotation given, is an adverbial form from the two-ending adjective megalomerh/s, -e/s, consisting of large particles, lavish, magnificent; see generously LSJ s.v.
[1] Polybius fr. 181 (Büttner-Wobst). Büttner-Wobst notes (538) that Toup attributed this fragment to Polybius. The fragment is unplaced. The individuals named are possibly the Roman general and consul Publius Cornelius Scipio Africanus (236-183 BCE; cf. OCD(4) p.382, Walbank (191ff), and pi 2054) and Gaius Laelius (c.235-c.160; cf. OCD(4) s.v. Laelius(1) and Walbank (198-200)), who was a friend from childhood and served as a general for Scipio Africanus in Iberia; but the individual who treats them in the way described is unidentifiable. For a passage with very similar vocabulary see Polybius 23.2.9 (web address 1).