Perfidus, has the same etymological meaning than "faithless" (not with the same roots)
perfidus: per + fides.
Fides = faith (trust)
Possible meaning of "perfidus" in English:
That breaks his promise; faithless, false, dishonest, treacherous, perfidious, deceitful.
(by extension) Treacherous, unsafe, dangerous.
So, perfidious, treacherous, dishonest, and deceitful,
should probably all be accepted.
Perfid, from Perfidy.
Borrowed to French Perfidie. (I don't know the exact meaning in old French, but seems the same than nowadays, like in "Perfide Albion".
It's interesting to consider the go-between language, as it could change the meaning from Latin to English.
Perfidie: From Latin perfidia "faithlessness, falsehood, treachery," from perfidus "faithless," from phrase per fidem decipere "to deceive through trustingness," from per "through" (from PIE root per- (1) "forward," hence "through") + fidem (nominative fides) "faith" (from PIE root bheidh- "to trust, confide, persuade"). Perfidie: to be perfidious, treacherous