That title feels extra obscure even though I totally explain it below. Weird.

草を打って蛇を驚かす
(Kusa wo utte hebi wo odorokasu; “Strike the grass, surprise a snake”)

Definition:

An innocent or unthinking action that draws unintended, usually bad, consequences. Like idly swatting at the long grass by the roadside only to startled and draw out a snake that thought you were attacking it.

Alternately, to single out and punish one wrongdoer as a warning others, especially others who have some connection to the first.

Breakdown:

We begin with the noun 草 (kusa), “grass,” marked as the object of a verb by the particle を (wo). The verb being done to the grass is 打つ (utsu), in conjunctive form so that it can connect to the second half of the sentence. In turn, this begins with 蛇 (hebi), “snake.” The snake is also marked by を, and the verb being done to it is 驚く (odoroku), “to be surprised,” in causative-passive form. (I.e. the snake is being caused to become surprised.)

Notes:

This saying comes from the Chinese, and can also be found in yojijukugo form as 打草驚蛇. It’s attested in a Song-era dictionary of idioms called the 書言故事 (Shogen koji), in the story of a public official named 王魯 – I’m told it’s pronounced Wang Lu – one of whose subordinates is charged with taking bribes. 王魯 himself was guilty of the same, but takes the hint and declares himself to be like a snake taking fright even though the grass nearby, rather than the snake itself, was struck.

This implies that the second usage listed above is the original and “proper” one. It’s not clear how the “bad consequences” meaning developed, although I’d guess it’s from people seeing the phrase and guessing at its meaning without knowing its origin or common usage.

The cause and recipient of the surprise can also be reversed by replacing the latter half of the saying with 蛇に驚く (hebi ni odoroku), “be surprised by a snake.” Another similar saying replaces the grass with “underbrush,” 藪 (yabu).

Example sentence:

「草を打って蛇を驚かすつもりで、あの汚職官吏の補佐官まで指摘して処罰を与えてみたが、さすがナルシストか、空気を読まずいつも通りに続けた」

(“Kusa wo utte hebi wo odorokasu tsumori de, ano oshoku kanri no hosakan made shiteki shite shobatsu wo ataete mita ga, sasuga narushisuto ka, kuuki wo yomazu itsumo doori ni tsudzuketa.”)

[“As a warning shot we picked out the corrupt official’s aide for punishment, but I guess as you’d expect from a narcissist, he just carried on like always without taking the hint.”]