A sweater is a warm piece of clothing worn on the upper body, usually made of knitted wool or yarn. It is often used in cold weather to keep you warm.
From 'sweat' plus the agent noun suffix '-er', originally used for a garment worn to make you sweat during exercise. Over time it came to mean any knitted warm top.
The name 'sweater' hints that it was once workout gear meant to make you sweat, not cozy fashion. Clothing words often carry little fossils of old lifestyles we barely recognize today.
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