Lewis & Short

socrus, ūs (orig. comm.; v. infra), f. [a collat. form of socer; Gr.ἑκυρός], orig.also m., either a father-in-law or a mother-in-law; but of the first signif. we have only two examples in ancient poets: praemia erepta a socru suo, Att. ap. Prisc. p. 698 P. (Trag. Rel. p. 136 Rib.); and: a socru, Oenomao rege, Poët. ap. Cic. Tusc. 3, 12, 26 (Rib. l. l. p. 213).
The signif. mother-in-law was the prevailing one through all periods of the language: uno animo omnes socrus oderunt nurus, Ter. Hec. 2, 1, 4; 5, 1, 22; cf. Ov. F. 2, 626; Ter. Hec. 2, 3, 4; 4, 4, 83; Cic. Clu. 12, 23; Juv. 6, 231; Dig. 23, 2, 14, § 4.
Collat. form SOCERA, ae (acc. to socer), Inscr. Orell. 289; and contr. SOCRA, ib. 4221: magna, grandmother-in-law, i. e. one’s husband’s or wife’s grandmother: major, the great-grandmother of either party, Dig. 38, 10, 4, § 6; Fest. p. 126 Müll.