ETYM Latin perfidious.
1. Guilty of perfidy; violating good faith or vows; false to trust or confidence reposed; teacherous; faithless.
2. Involving, or characterized by, perfidy.
Revealing; that which gives signs of what one might to have concealed.
ETYM Cf. French traîtreux.
Constituting treason; SYN. treasonable, treasonous.
Dangerously unstable and unpredictable; SYN. unreliable.
Relating to, consisting of, or involving treason
Disloyally, treacherously
In a treacherous manner.
In a treasonable manner, in a manner that is related to betrayal or breach of faith