Lewis & Short

Sīpontum, i, n., = Σιποῦς (cf. Cic. Att. 6, 2, 3), an important maritime town in Appulia, now S. Maria di Siponto, Mel. 2, 4, 7; Plin. 3, 11, 16, § 103; Cic. Att. 9, 15, 1; 10, 7, 1; Liv. 34, 45, 3; 39, 23, 3 al. (in Liv. 8, 24, 4 dub., v. Weissenb. ad h.l.).
In a Greek form, Sīpūs, Sil. 8, 635; Luc. 5, 377: Sipontum, aut ut Graii dixere, Sipus, Mel. 2, 4, 7.
Hence, Sīpontīnus, a, um, adj., of or belonging to Sipontum, Sipontian: siccitas, Cic. Agr. 2, 27, 71: POPVLVS, Inscr. Murat. p. 1081, 6.