An Etymological Dictionary of the German Language/Annotated/fett
fett, adjective, ‘fat, plump,’ only in Modern High German, introduced by Luther from Middle German and Low German instead of the genuine Upper German feist; Low German fett, compare Dutch vet from an earlier fêtt, Anglo-Saxon fœ̂tt, ‘fat,’ which, with Old High German feiȥȥit, are derived from Gothic *faitiþs; see feist. As to the origin of the Modern High German idiom, sein Fett haben, jemandem sein Fett geben, ‘to get one's due, give any one his due,’ opinions are divided; although the reference to einbrocken, jemandem etwas einbrocken (to play one a trick), &c., supports the assumption of a purely German origin, some etymologists regard it as partly translated and partly borrowed from the French donner à quelqu'un son fait, avoir son fait, others even as an ironical reference to the French faire fête à quelqu'un, ‘to make a person heartily welcome.’