The Collaborative International Dictionary of English v.0.48:

Tetanic \Te*tan"ic\, a. [Cf. L. tetanicus suffering from
   tetanus, Gr. ?, F. t['e]tanique.]
   1. (Physiol.) Of or pertaining to tetanus; having the
      character of tetanus; as, a tetanic state; tetanic
      contraction.
      [1913 Webster]

            This condition of muscle, this fusion of a number of
            simple spasms into an apparently smooth, continuous
            effort, is known as tetanus, or tetanic contraction.
                                                  --Foster.
      [1913 Webster]

   2. (Physiol. & Med.) Producing, or tending to produce,
      tetanus, or tonic contraction of the muscles; as, a
      tetanic remedy. See Tetanic, n.
      [1913 Webster]
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Tetanic \Te*tan"ic\, n. (Physiol. & Med.)
   A substance (notably nux vomica, strychnine, and brucine)
   which, either as a remedy or a poison, acts primarily on the
   spinal cord, and which, when taken in comparatively large
   quantity, produces tetanic spasms or convulsions.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tetanic
    adj 1: of or relating to or causing tetany
    2: of or relating to or causing tetanus