"faithless, basely treacherous," 1590s, from Latin perfidiosus "treacherous," from perfidia "faithlessness" (see perfidy). Related: Perfidiously; perfidiousness.
The treacherous man either betrays the confidence that is reposed in him, or lures another on to harm by deceitful appearances; as, the treacherous signals of the wrecker. The perfidious man carries treachery to the basest extreme: he betrays acknowledged and accepted obligations, and even the most sacred relationships and claims: as, Benedict Arnold and Judas are types of perfidy. [Century Dictionary, 1895]