1. the quality or state of being unfaithful, disloyal, or treacherous; deliberate breach of trust or faith
Usage: formal; typically used to describe serious betrayal or disloyalty

Examples

• The general was shocked by the perfidiousness of his trusted advisor, who had secretly negotiated with the enemy.
• Her perfidiousness became apparent when she revealed confidential information to her rival.
• The treaty was broken due to the perfidiousness of one of the signatory nations.
• He could not forgive the perfidiousness of his former business partner.
• The novel explores themes of perfidiousness and redemption among its characters.
• Such perfidiousness in a friendship is difficult to overcome.