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Phrase(s): *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.
McGrawhill's American Idioms And Phrasal Verbs