in the bull pen
*in the bull pen
1. Lit. [of a baseball pitcher to be] in a special place near the playing field, warming up to pitch. (*Typically: be ~; go [into] ~.) You can tell who is pitching next by seeing who is in the bull pen. Our best pitcher just went into the bull pen. He'll be pitching soon.
2. Fig. in reserve, ready if needed. I'm willing to be in the bull pen. Just call me if you need me.