OXFORD DICTIONARY
duff, n. a boiled pudding.
duff, adj. Brit. sl.
1 worthless, counterfeit.
2 useless, broken.
duff, v.tr. sl.
1 Brit. Golf mishit (a shot, a ball); bungle.
2 Austral. steal and alter brands on (cattle).
duff up sl. beat; thrash.
perh. back-form. f. DUFFER
THESAURUS
duff
arse, bad, beat, behind, bilk, bottom, broken, butt, can, cheat, counterfeit, cozen, defraud, diddle, do, dud, fake, false, fanny, gyp, inoperable, inoperative, overreach, prat, rear, screw, tail, unworkable, useless, worthless