The Collaborative International Dictionary of English v.0.48:

Duff \Duff\ (d[u^]f), v. t. [imp. & p. p. Duffed; p. pr. & vb.
   n. Duffing.] [Etym. uncertain.] [Colloq. or Slang]
   1. To treat or manipulate so as to give a specious appearance
      to; to fake; hence, to cheat.
      [Webster 1913 Suppl.]

   2. In Australia, to alter the brands on (cattle, horses,
      etc.); to steal (cattle, etc.), and alter their brands.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Duff \Duff\ (d[u^]f), n. [From OE. dagh. [root]67. See Dough.]
   1. Dough or paste. [Prov. Eng.] --Halliwell.
      [1913 Webster]

   2. A stiff flour pudding, boiled in a bag; -- a term used
      especially by seamen; as, plum duff.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Duff \Duff\ (d[u^]f), n.
   the buttocks; as, get off your duff and get to work. [slang]

   Syn: rump; ass. []

WordNet (r) 3.0 (2006):

duff
    n 1: a stiff flour pudding steamed or boiled usually and
         containing e.g. currants and raisins and citron [syn:
         duff, plum duff]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "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



The Free On-line Dictionary of Computing (30 December 2018):

duff

   1.  Duff's device.

   2.  Tom Duff.