Lewis & Short

sescuncĭa (SESCONCIA, Inscr. Orell. 4563), ae, f. [sesqui-uncia], one and a half unciae, i.e. a twelfth and a half, = one eighth of a whole, Plin. 36, 25, 62, § 187; Cels. 5, 18, 28; Col. 12, 59, 4; Scrib. Larg. 50; 60; Front. Aquaed. 26; Dig. 37, 8, 7 fin.
As adj.: copulae sescunciae, an inch and a half thick, Plaut. Ep. 5, 1, 11; cf. the foll. art.