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.]