The Collaborative International Dictionary of English v.0.48:

Piacle \Pi"a*cle\, n. [L. piaculum a propitiatory sacrifice,
   that which requires expiation, a wicked action, fr. piare to
   appease, to expiate, pius pious.]
   A heinous offense which requires expiation. [R.] --Howell.
   [1913 Webster]