风吹草动

fēng chuī cǎo dòng

the slightest sign of trouble

Character by character

风fēng
wind

吹chuī
blows

草cǎo
grass

动dòng
stirs

股市一有风吹草动，他就紧张。

The slightest stir in the stock market makes him nervous.

Tone: neutral.

Usually 一有风吹草动 + reaction.

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