The Collaborative International Dictionary of English v.0.48:

Rap \Rap\, n. [Perhaps contr. fr. raparee.]
   A popular name for any of the tokens that passed current for
   a half-penny in Ireland in the early part of the eighteenth
   century; any coin of trifling value.
   [1913 Webster]

         Many counterfeits passed about under the name of raps.
                                                  --Swift.
   [1913 Webster]

         Tie it [her money] up so tight that you can't touch a
         rap, save with her consent.              --Mrs.
                                                  Alexander.
   [1913 Webster]

   Not to care a rap, to care nothing.

   Not worth a rap, worth nothing.
      [1913 Webster]