From eu and onoma; properly, well-named (good-omened), i.e. The left (which was the lucky side among the pagan Greeks); neuter as adverbial, at the left hand -- (on the) left.
see GREEK eu
see GREEK onoma
1. of good name (Hesiod, Pindar), and of good omen (Plato, polit., p. 302 d.; legg. 6, p. 754 e.); in the latter sense used in taking auguries; but those omens were euphemistically called εὐώνυμα which in fact were regarded as unlucky, i. e. which came from the left, sinister omens (for which a good name was desired); hence,
2. left (so from Aeschylus and Herodotus down): Acts 21:3; Revelation 10:2; ἐξ εὐωνύμων (cf. Winer's Grammar, § 27, 3; § 19 under the word δεξιά; Buttmann, 89 (78)), on the left hand (to the left): Matthew 20:21, 23; Matthew 25:33, 41; Matthew 27:38; Mark 10:37 (R G L),