Euphemistic adjective (here in the neuter nominative/accusative singular, evidently quoted from somewhere: see n.1 below), "because to a Greek augur, looking northward, the unlucky signs came from the left" (LSJ).
For the 'well-named' (left) wing of an army in battle-order see delta 1304, pi 315, etc.
On words for 'left," see also lambda 174, lambda 193.
[1] Same glossing in Hesychius, where Latte claims the headword itelf as quoted from the New Testament (Acts 21.3, a sea voyage passing Cyprus on the left).