The Collaborative International Dictionary of English v.0.48:

Headshake \Head`shake`\ (-sh[=a]k`), n.
   A significant shake of the head, commonly as a signal of
   denial. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

headshake
    n 1: the act of turning your head left and right to signify
         denial or disbelief or bemusement; "I could tell from their
         headshakes that they didn't believe me" [syn: headshake,
         headshaking]