The modern English meaning of "perfidious" remains faithful to that of its Latin ancestor, perfidus, which means "faithless." English speakers have used "perfidious" to mean "treacherous" since at least 1572.

What is the dictionary definition of perfidious?

deliberately faithless; treacherous; deceitful: a perfidious lover.

What is synonym for treacherous?

Some common synonyms of treacherous are disloyal, faithless, false, perfidious, and traitorous.