Lewis & Short

sōbrīnus, i, m., and sōbrīna, ae, f. [contr. for sororinus from soror, and therefore prop. an appellation for the children of sisters; hence, in gen.], a cousin-german, cousin by the mother’s side, Fest. p. 297 Müll.; Dig. 38, 10, 3; Just. Inst. 3, 6.

        1. (α) Masc., Ter. And. 4, 5, 6; id. Phorm. 2, 3, 37; Cic. Off. 1, 17, 54; Dig. 38, 10, 1; 38, 10, 10.
        2. (β) Fem., Plaut. Poen. 5, 2, 108; Tac. A. 12, 6 and 64; Dig. 38, 10, 10.