Source : Webster's Revised Unabridged Dictionary (1913)

Opisthotonos \Op`is*thot"o*nos\, n. [NL., from Gr. ? backwards +
   ? a stretching.] (Med.)
   A tetanic spasm in which the body is bent backwards and
   stiffened.

Source : WordNet®

opisthotonos
     n : severe spasm in which the back arches and the head bends
         back and heels flex toward the back