A substitute player in sports who spends most of the game sitting on the bench instead of playing, or someone who doesn't actively participate in an activity.
From 'bench' (where substitute players sit) and 'warmer' (one who warms something by sitting). This sports slang emerged in American English in the early 20th century.
Being called a 'benchwarmer' stings in sports, but it's actually a humorous term—the image of literally warming a bench with your body heat shows how sports slang is born from vivid physical metaphors!
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