Lewis & Short

Symaethum, i, n., or Symaethus i, m., = Σύμαιθος, a river and town situated upon it on the east coast of Sicily, near Catina, Plin. 3, 8, 14, § 89; Sil. 14, 231; 9, 410; Serv. ad Verg. A. 9, 584.
Hence,

  1. A. Sȳ̆-maethĭus, a, um, adj., of or belonging to Symæthus, Symæthian: flumina, Verg. A. 9, 584: heros, i. e. Acis, son of the nymph of the Symæthus, Ov. M. 13, 879.
    As subst.: Symaethĭi, ōrum, m., the dwellers on the Symæthus, Plin. 3, 8, 14, § 91.
  2. B. Symae-thēus, a, um, adj., Symæthian: aquae, Ov. F. 4, 472.
  3. C. Symaethis, ĭdis, f., Symæthian: nympha, Ov. M. 13, 750.