According to Adler, both this same entry and a comparable one occur in the Ambrosian Lexicon.
The headword means someone or something with a beard, whether person, animal, or comet. See generally LSJ s.v., citing instances from Cratinus onwards. (For beards see also pi 2150, pi 2152.)
The present gloss does not seem quite apt. Küster emended to o( me/gan pw/gwna e)/xwn "he who has a big beard"; Bernhardy, to megalopw/gwn "big-bearded" (an adjective attested once in John Malalas).