This word attracted much commentary.
Heraclides says that it was an Ionian and Aeolian dialect form (fr. 19 line 47). Herodian (
De Prosodia Catholica 3,1. 510.6) lists it among adverbs ending in -x that have the oxytone (acute) accent on the final syllable (cf.
epsilon 2807).
It is used twice in
Homer of a warrior who attacks unseen from the blind side of his enemy (
Iliad 11.251, 15.541: web addresses 1 and 2). As W. Leaf saw long ago in his edition of the
Iliad (ad 11.251), it is a military term, referring to an outflanking maneuver -- or, in naval warfare, to an attack to ram the exposed broadside of a ship. (NB: these two contexts are reflected in the suggested translation of the headword, above, but Prof J C McKeown points out that in rugby they are opposites.)
The jingle used by Pisthetaerus [probably better Pisetaerus as in Dunbar and other editions] to Iris the Rainbow in
Aristophanes,
Birds 1258 (web address 3),
eu)ra/c pata/c 'On the blind side! Ram!' (
epsilon 2807 note 10), appears to reflect a proverb or command on seeing an enemy exposed to attack on the blind side or broadside. In context it counts as sexual harassment, and Iris threatens to report it as hybris to her father Zeus.
The etymology is unclear. Risch hesitantly follows the majority of
scholia, lexica and commentaries in deriving it, by analogy with other adverbs in -ax, from
eu)ru/s 'wide (as the horizon stretching before us is wide)' and
eu)=ros 'width' (
Wortbildung der homerischen Sprache, p. 308, §128 a). Tichy, followed by
Lexikon des frühgriechischen Epos fasc. 12. 792, is even less certain.