"breach of faith or trust, base treachery," 1590s, from Middle French perfidie (16c.), 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").
[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"]
