perfidious
treacherous or deliberately faithless
Roots
Other words with the fid root
- confidence: trust in oneself or something
- diffidence: lack of self-confidence
- diffident: lacking confidence
- faith: trust or belief kept
- fidelity: faithfulness or loyalty
- perfidy: treachery or breach of faith
Perfidious means faith broken on purpose: trust is used and then betrayed.
The perfidious ally promised support while secretly funding the enemy.