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Definition of perfidious : 1. Guilty of perfidy; violating good faith or vows; false to trust or confidence reposed; teacherous; faithless; as, a perfidious friend.
2. Involving, or characterized by, perfidy.

Synonyms:

continue, Punic, recreant, false, trust, traitorous, faithless, untrue, unfaithful, disloyal, corrupt, study at faithless, unreliable, false-hearted, dishonest

black-hearted, ill-natured, hurtful, malicious, hateful, sullen, evil, malignant, malevolent, rancorous, venomous, corrosive, virulent, spiteful, antagonistic, nasty, catty, wicked, sinister, churlish, bitter, baleful, treacherous, vicious, mean, vindictive, foul, baneful, hostile

Usage examples:

• In answer to a petition made by the States against the appointment of York, " in respect to his perfidious dealings before," the Earl replied that he would answer for his fidelity as for his own brother; adding peremptorily-" Do you trust me? - "History of the United Netherlands, 1586-89, Vol. II. Complete", John Lothrop Motley Last Updated: February 7, 2009.
• And that perfidious Marquis, Sir, raised his eyes and looked me squarely- yes! - "Castles in the Air", Baroness Emmuska Orczy.
• We humbly conceive that it is contrary to the Maxims of good Policy and extremely dangerous to our Frontiers, to suffer any Indians, of what Tribe soever, to live within the inhabited Parts of this Province, while we are engaged in an Indian War, as Experience has taught us that they are all perfidious, and their Claim to Freedom and Independency, puts it in their Power to act as Spies, to entertain and give Intelligence to our Enemies, and to furnish them with Provisions and warlike Stores. - "The Conspiracy of Pontiac and the Indian War after the Conquest of Canada", Francis Parkman.