From Longman Dictionary of Contemporary Englishduffduff1 /dʌf/ noun [countable] 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!
Examples from the Corpus
duff• Even if she didn't spot that her sister was up the duff, she must have known her mum wasn't.duffduff2 adjective British English informal BROKENbad or uselessExamples from the Corpus
duff• A Godsend for anyone who has to submit work as it gets rid of the chance of sending a duff disk.• This has been a duff year for him: the runs have flowed like treacle, and Dame Fortune has turned sour.duffduff3 verb → duff somebody ↔ up→ See Verb tableExamples from the Corpus
duff• For a small fee, usually a week's tuck allowance, she would duff up any victim selected.Origin duff1 (1800-1900) dough duff2 (1800-1900) Perhaps from duffer 