ラテン語のperfidia、裏切り、不誠実、per-, whole, -fid, to believe, trustに由来し、語源的には信仰、信頼と同じ。perjuryと比較される。
[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"]