perfidious
Definitions
Deceitful and untrustworthy.
Characterized by a deliberate breach of faith or betrayal of trust.
Examples
The perfidious knight betrayed his king by opening the castle gates to the enemy army.
She could not forgive his perfidious behavior after he leaked her private documents to the press.
Synonyms
treacherous deceitful disloyal faithless traitorous untrustworthy duplicitous