warm the bench
warm the bench {v. phr.}, {informal}
To act as a substitute on an athletic team.
Bill has been warming the bench for three football seasons; he hopes that the coach will let him play this year.
- bench warmer {n.}, {informal}
A substitute player.
Last year Ted was only a bench warmer, but this year he is the team's star pitcher.
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Source: A Dictionary of American Idioms