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Phrase(s): warm the bench
Fig. [for a player] to remain out of play during a game—seated on a bench. • John spent the whole game warming the bench. • Mary never warms the bench. She plays from the beginning to the end.
McGrawhill's American Idioms And Phrasal Verbs
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Also, ride the bench. Be a secondary or substitute participant; wait one's turn to participate. For example, I can't wait till the head of accounting retires; I've been warming the bench for years. This expression comes from such sports as baseball and football, and their standard practice of having substitute players sit on a bench in case they are needed in a game. [Slang; early 1900s]
American Heritage Idioms