I. duff 1 /dʌf/ BrE AmE noun [countable]
[ Date: 1800-1900 ; Origin: dough ]
American English informal the part of your body that you sit on SYN bottom :
Get off your duff (=stop sitting or stop being lazy) and help me!
II. duff 2 BrE AmE adjective British English informal
[ Date: 1800-1900 ; Origin: Perhaps from duffer ]
bad or useless
III. duff 3 BrE AmE verb
duff somebody ↔ up phrasal verb British English informal
to fight someone and injure them