1570s, "the lash of a whip," from whip (n.) + lash (n.). The injury caused by sudden head motion so called by 1955, in reference to the notion of moving to and fro like a cracking whip. The verb in this sense is recorded by 1971.
whiplash injury n. A hyperextension-hyperflexion injury to the cervical spine caused by an abrupt jerking movement of the head, either in a backward or forward direction.
whiplash whip·lash (wĭp'lāsh')n. Whiplash injury.