Involuntary trembling movements of the body from cold or fear, or to shake with these tremors.
From Middle English 'chivere,' possibly from Old English 'ceafl' meaning jaw. Originally described chattering teeth; expanded to describe full-body trembling.
Shivers actually serve a purpose—your muscles shaking generate heat when you're cold, and the same response evolved to help you become 'dangerous-looking' when scared, making your body bigger.
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