It implies:
* No physical movement: The person is not even twitching or blinking.
* Lack of reaction: They are not showing any signs of emotion, surprise, or even interest.
* Passive observation: They might be watching something or listening to something, but they are not actively participating.
Here are some examples of how the idiom might be used:
* "He sat through the entire lecture without moving a muscle, listening intently to every word."
* "The cat watched the bird outside the window, completely still, without moving a muscle."
* "She was so surprised by the news that she just stood there, without moving a muscle."
The idiom often suggests a state of calm, concentration, or even shock. It can also be used humorously to describe someone who is being exceptionally stoic or unfazed.