The Collaborative International Dictionary of English v.0.48:

dibs \dibs\ (d[i^]bz), n.
   1. A sweet preparation or treacle of grape juice, much used
      in the East. --Johnston.
      [1913 Webster]

   2. A small amount of money.
      [PJC]

   3. A right or claim, such as the right of use, or of first
      choice; as, I have dibs on the binoculars after Jim is
      finished..
      [PJC]

WordNet (r) 3.0 (2006):

dibs
    n 1: a claim of rights; "I have dibs on that last slice of
         pizza"