不动声色

bù dòng shēng sè

without showing any emotion

Character by character

不bù
not

动dòng
move

声shēng
voice

色sè
face

他不动声色地把问题解决了。

He quietly fixed the problem without any fuss.

Tone: neutral.

Keeping a straight face, or acting quietly so nobody notices.

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This idiom and thousands more are in Learn Chinese with Bao, with audio, pinyin and drills that make them stick.