perfidious

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

Guilty of perfidy; violating good faith or vows; false to trust or confidence reposed; teacherous; faithless; as, a perfidious friend.

Latin perfidus, from per- + fidus, faithful

“Perfidious Snake-plants had strangled what I knew Was a pavilion once: each oak Held on his horns some spoil he broke By surreptitiously beneath Upthrusting: pavements, as with teeth, Griped huge weed widening crack and split In squares and circles stone-work erst.”

— Robert Browning, The Complete Poetic and Dramatic Works of Robert Browning Cambridge Edition

Sofia, a Karachi street vendor, trusted her neighbor to watch her stall while she dashed home for lunch. Upon returning, she found her stall empty and her neighbor grinning widely. "Perfidious snake!" Sofia yelled, chasing after him with her biggest frying pan.

Thandi sat at the waterfront in Wellington, sipping her flat white, when her friend Sam promised to buy her a new camera if she lent him hers. Moments later, he handed over an old disposable. Thandi's eyes narrowed as she realized how perfidious his smile had been under the city’s drizzly sky.

“She is trembling, she is fainting, perfidious souls, deaf to pity!" The soldiers continue to demand Arturo's execution, but the sounds of a herald arriving are heard.”