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.
Many counterfeits passed about under the name of raps.
Tie it [her money] up so tight that you can't touch a
rap, save with her consent. --Mrs.
Not to care a rap, to care nothing.
Not worth a rap, worth nothing.