köken: Etymology tree Proto-Indo-European *dewh₂-der.? Proto-Italic *dwenos Old Latin duenos Old Latin duonus Latin bonus Old French bon Middle French bon French bon Inherited from Middle French bon, from Old French bon, from Latin bonus (“good”), from Old Latin duenos, later duonus, from Proto-Italic *dwenos.
aile: à bon chat, bon rat, à bon compte, à bon entendeur salut, à bon escient, à bon vin point d'enseigne, à la bonne franquette, à la bonne heure, à méchant ouvrier, point de bon outil, à quelque chose malheur est bon, à quoi bon
köken: From the gerund of tagliare, meaning a small strip of paper to be separated from a main one. Car service sense is from a time where the procedure was annotated on such a strip. Compare French coupon from couper (“to cut”) for a similar semantic shift.