1. deliberately unfaithful; treacherous
Usage: formal

Examples

• The perfidious ally betrayed the treaty at the crucial moment.
• She discovered her perfidious friend had been spreading rumors about her.
• The novel’s villain was a perfidious character who deceived everyone around him.
• His perfidious behavior cost him the trust of his business partners.
• The perfidious act of selling state secrets shocked the nation.
• Many considered the ambassador’s actions perfidious and unforgivable.