The Collaborative International Dictionary of English v.0.48:

Clonic \Clon"ic\, a. [Gr. klo`nos a violent, confused motion;
   cf. F. clonique.] (Med.)
   Having an irregular, convulsive motion. --Dunglison.
   [1913 Webster]

   Clonic spasm. (Med.) See under Spasm.
      [1913 Webster]