The primary headword, in the genitive case, is clearly quoted from somewhere, but the known instances of megalorrh/monos outside lexica and grammars are not promising: it occurs in John Damascene, Sacra Parallela PG 95.1285 (as a title), and in Sophronius of Jerusalem, Miracles of SS Cyrus and John 9 line 60 Fernández Marcos.
The related noun megalorhmosu/nh is also mentioned, unglossed, in mu 376 with the classical spelling megalorrhmosu/nh (the Suda is generally indifferent to single and double consonants). It is first recorded in 1 Samuel 2:3 and Odes 3:3 LXX. Hesychius mu475 defines it instead as mega/la, lalo/nwn (of those uttering grandly?).