perfidious
- Guilty of betraying trust or breaking faith; deceitfully disloyal.
In a sentence
- The general was shocked when his most trusted advisor proved perfidious, leaking battle plans to the enemy.
- The phrase 'perfidious Albion' became a long-standing expression of distrust toward British foreign policy.
Usage note. Perfidious is formal and literary in register; it implies a calculated, deliberate betrayal rather than a casual lapse in loyalty.
Where it comes from
English perfidious · entered English late 16th c.
From Latin 'perfidiosus', derived from 'perfidia' (treachery), itself from 'perfidus' (faithless), combining 'per-' (through, away) and 'fides' (faith). The word entered English in the late 16th century, largely through literary and political writing of the Renaissance period.