Extremely silent or making very little noise, often used to describe someone moving stealthily or being unusually subdued.
This simile has been used in English since at least the 16th century, drawing on the mouse's reputation for moving silently to avoid predators. The comparison emphasizes both the practical necessity of quietness for survival and the almost supernatural ability of mice to move undetected.
The mouse comparison is particularly apt because these small creatures have evolved to be virtually soundless when moving - their soft paws, light weight, and careful movements make them masters of stealth. The phrase captures not just quietness but also the sense of vulnerability and caution that makes such silence necessary, whether for a mouse avoiding cats or a person trying not to wake someone.
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