This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

adj. Of, relating to, or marked by perfidy; treacherous. See Synonyms at faithless.

adj. Of, pertaining to, or representing perfidy; disloyal to what should command one's fidelity or allegiance.

adj. Guilty of perfidy; violating good faith or vows; false to trust or confidence reposed; teacherous; faithless.

adj. Involving, or characterized by, perfidy.

Faithless; basely treacherous; false-hearted.

Proceeding from or characterized by perfidy or base treachery; false: as, a perfidious act.

adj. tending to betray; especially having a treacherous character as attributed to the Carthaginians by the Romans

"We wouldn't lie to you about the history of "perfidious" -- even though the word itself suggests deceitfulness."

unfaithful   

perfidiously    perfidiousness   

teacherous    faithless    unfaithful    treacherous    traitorous   

faithless    traitorous    treacherous    tyrannical    avaricious    ungrateful    deceitful    crafty    bloodthirsty    despicable