Lewis & Short

1. Ācis, ĭdis, m., = Ἆκις,

  1. I. a river in Sicily, which rises in Mount Aetna, and falls into the sea; now Fiume di Taci, Ov. F. 4, 468; Sil. 14, 221; Claud. Rapt. Pros. 3, 332 al.
    Hence,
  2. II. A river-god, acc. to the myth, son of Faunus, beloved by Galatea on account of his beauty, Ov. M. 13, 750 sq.