move a muscle
move a muscle {v. phr.}
To move even a very little. — Used in negative sentences and questions and with "if".
The deer stood without moving a muscle until the hunter was gone.
The girls were so startled that they did not move a muscle.
You're sitting right where you were when I left! Have you moved a muscle?
The robber said he would shoot the bank worker if he moved a muscle.
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Source: A Dictionary of American Idioms
