1550s, "a written challenge, letter of defiance," from French cartel (16c.), from Italian cartello "placard," diminutive of carta "card" (see card (n.1)).
It came to mean "written agreement between states at war" (1690s), for the exchange of prisoners or some other mutual advantage, then "a written agreement between challengers" of any sort (1889). Sense of "a commercial trust, an association of industrialists" is from 1900, via German Kartell, which is from French. The older U.S. term for that is trust (n.). The usual German name for them was Interessengemeinschaft, abbreviated IG.