The Collaborative International Dictionary of English v.0.48:

Throwing stick \Throw"ing stick`\ (Anthropol.)
   An instrument used by various savage races for throwing a
   spear; -- called also throw stick and spear thrower. One
   end of the stick receives the butt of the spear, as upon a
   hook or thong, and the other end is grasped with the hand,
   which also holds the spear, toward the middle, above it with
   the finger and thumb, the effect being to bring the place of
   support nearer the center of the spear, and practically
   lengthen the arm in the act of throwing. In Mexico, one such
   device is called the atlatl.
   [Webster 1913 Suppl. +PJC]
   [1913 Webster]

WordNet (r) 3.0 (2006):

spear thrower
    n 1: a device resembling a sling that is used in various
         primitive societies to propel a dart or spear [syn:
         throwing stick, throwing board, spear thrower, dart
         thrower]