Poetic adjective of uncertain dialectal provenance (see LSJ s.v., web address 1), here in the nominative/accusative neuter singular, with the same or similar glossing in other lexica. It must be quoted from somewhere: probably either Aeschylus (several instances; the likeliest is line 102 of the Prometheus Bound, which may or may not be Aeschylean) or Pindar (Olympians 10.60).