The Collaborative International Dictionary of English v.0.48:

Piacular \Pi*ac"u*lar\, a. [L. piacularis: cf. F. piaculaire.]
   1. Expiatory; atoning. --Sir G. C. Lewis.
      [1913 Webster]

   2. Requiring expiation; criminal; atrociously bad. "Piacular
      pollution." --De Quincey.
      [1913 Webster]