means A knitted garment for the upper body, usually with sleeves, worn for warmth.

from From the verb 'sweat,' which traces back to Old English 'swǣtan,' to perspire. A 'sweater' was originally something that made you sweat — in the 19th century the word named a heavy woolen garment athletes wore to work up a sweat and shed weight before a match. Over time the heat-inducing punishment became the cozy comfort, and 'sweater' settled into the soft, warm thing we now reach for against the cold.