An Etymological Dictionary of the German Language/fett
fett, adj., ‘fat, plump,’ only in ModHG., introduced by Luther from MidG. and LG. instead of the genuine UpG. feist; LG. fett, comp. Du. vet from an earlier fêtt, AS. fœ̂tt, ‘fat,’ which, with OHG. feiȥȥit, are derived from Goth. *faitiþs; see feist. As to the origin of the ModHG. 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 Ger. origin, some etymologists regard it as partly translated and partly borrowed from the Fr. donner à quelqu'un son fait, avoir son fait, others even as an ironical reference to the Fr. faire fête à quelqu'un, ‘to make a person heartily welcome.’