warm the bench - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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]