A sudden jerking motion of the neck, typically from a car accident, that causes pain and injury; also means a sudden shock or unexpected change.
From 'whip' (to strike with a lashing motion) plus 'lash' (a striking blow). The term became common for neck injuries in the 1900s as car accidents increased, though the image comes from actual whip injuries.
Whiplash injuries are tricky because damage to soft tissues (ligaments and tendons) doesn't show up on X-rays, so people with real whiplash were sometimes dismissed as faking until MRI technology proved the injuries were real!
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