Lewis & Short

Cătădūpa, ōrum, n., = Κατάδουπα, the celebrated cataract of the Nile, near Syene, on the borders of Egypt (cf. cataracta), now Chellal, Cic. Rep. 6, 18, 19; Macr. Somn. Scip. 2, 4, § 14.
Cătădūpi, ōrum, m., those dwelling near, Plin. 5, 9, 10, § 54; 6, 29, 35, § 178; Amm. 22, 15, 2.