1. Sharp or caustic in style, tone, sarcastic, etc.: Chuck spoke in a very mendacious manner to his sister when he caught her nosing about in his desk.
2. Concerning the capability of wounding; barbed, biting, nipping, pungent: Such mordacious ways of communication can be a barbed compliment, a biting aphorism, or a pungent satire.
3. Referring to someone or animal that is given to biting: Mr. Smith deliberately gave his rival a skittish and mordacious horse for the outing.