To make maximum effort or use all available resources to achieve something, holding nothing back.
This phrase comes from pipe organs, where 'stops' are knobs that control which sets of pipes produce sound. When an organist 'pulls out all the stops,' they engage every rank of pipes, creating the fullest, most powerful sound possible. The metaphorical use emerged in the mid-1800s.
The phrase beautifully illustrates how musical terminology often becomes metaphorical language - we also say someone 'strikes a chord' or 'orchestrates' a plan. A full pipe organ with all stops pulled can produce sounds ranging from whisper-quiet flutes to thunderous bass notes that can literally shake a cathedral, making it a perfect metaphor for unleashing one's full power.
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