expression, suru verb (irregular)
not budge an inch; not move at all
Fixed expression used almost exclusively in the negative to emphasize that someone or something does not move even slightly. Often used with ～ない.
彼は微動だにしなかった。
He didn't budge an inch.
その猫は微動だにせずにじっとしていた。
The cat stayed perfectly still, not moving a muscle.
The expression combines 微動 (slight movement) with the particle だに (even) and する (to do), literally 'to do even a slight movement'. The particle だに is a classical emphatic form meaning 'even', and the phrase is now a fixed idiom used in negative contexts.