(a) Of persons or animals: deceitful, lying, treacherous; -- also as noun; (b) disloyal, seditious.

(a) Of speech, counsel, behavior, skill, etc.: intended to deceive, deceitful, fraudulent; (b) of things, merchandise: false, defective, adulterated; (c) of law: falsified, perverted.

(a) Of things, ideas, opinions, feelings, pleasures, the world, etc.: not to be depended on, unreliable, deceptive, misleading; (b) unreal, illusory; (c) med. & surg. hazardous; of an operation or a treatment: uncertain in its effect upon life or health; of a patient: uncertain of being helped or harmed (by a certain treatment).