Neuter singular of this adjective, with the same glossing in earlier lexica, e.g. Photius epsilon2184; and see also, post-Suda, Etymologicum Magnum 390.54. (Hesychius epsilon51 is comparable but expressed differently.) Applied strictly to oars four times in Homer, Odyssey (e.g. 11.121: web address 1), so, if the lexicographers mean by "from oarsmanship" that the word comes from the (poetic) vocabulary of oarsmanship, there is no problem. If, on the other hand, they are positing an etymological link between the headword eu)h=res and ei)resi/a, LSJ would disagree (web address 2), deriving the word from the root of a)rari/skw ('fit'). It is this interpretation that is reflected in the translation of the headword; however, although it would be a bit redundant, the word may indeed be derived from the root of 'oarsmanship', -er-, on the pattern of words like tri-h/rhs ('trireme'), and a(lih/rhs ('sea-rowing', applied to an oar in Euripides, Hecuba 455 (web address 3)); cf. Chantraine, s.v. -h/rhs.
For the gloss see epsiloniota 182.