Source : Webster's Revised Unabridged Dictionary (1913)

Duff \Duff\, 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.

   2. In Australia, to alter the brands on (cattle, horses,
      etc.); to steal (cattle, etc.), and alter their brands.