Lewis & Short

mordācĭtas, ātis, f. [mordax], the power of biting or stinging, mordacity (post-Aug.): urticarum foliis inest aculeata mordacitas, Plin. 21, 15, 54, § 91: vinosa, sharp, like sour wine, id. 21, 18, 72, § 120.
Transf., snappishness, biting severity in language, Cassiod. Hist. Eccl. 1, 12.