mor·da·cious (môr-d sh s). adj. 1. Given to biting; biting. 2. Caustic; sarcastic. [ Latin mord x, mord c- (from mord re, to bite; see mer- in Indo-European roots) + - ious.] mor·da cious·ly adv. mor·dac i·ty (-d s -t ) n. mordacious [mɔːˈdeɪʃəs]. adj ...