1590s, from M.Fr. perfidie, from L. perfidia "falsehood, treachery," from perfidus "faithless," from phrase per fidem decipere "to deceive through trustingness," from per "through" (see per) + fidem (nom. fides) "faith" (see faith). Related: perfidious (1590s).
"[C]ombinations of wickedness would overwhelm the world by the advantage which licentious principles afford, did not those who have long practiced perfidy grow faithless to each other." [Samuel Johnson, "Life of Waller"]