Whiplash is a neck injury caused by the head snapping back and then forward in a crash. Soft tissue, not bone.
It is the commonest injury in a rear-end collision and it often does not hurt until the following day, which surprises people. Britain reformed the law on whiplash claims after the number of them became implausible, which was unfair on the many people genuinely injured. The word comes from the crack of a whip, which is exactly the movement the neck makes.
1nounwhiplashA neck injury caused by a sudden jerking of the head.
“He was off work for a month with whiplash.”
From whip + lash.
Etymology from Wiktionary, CC BY-SA