The headword gives it in the genitive case; evidently quoted from somewhere but attested only here.
Such a vessel (muopa/rwn in Greek, myoparo in Latin) is attested from late-Hellenistic times onwards; cf. kappa 1404, and see generally Lionel Casson, Ships and Seamanship in the Ancient World (Baltimore & London 1995) 128, 132, 135, 168 n.58.