鳩が豆鉄砲を食ったよう
(Hato ga mamedeppou wo kutta you;
“Like a pigeon got shot by a pea-shooter”)

Definition:

Dazed; amazed; puzzled; left staring in shock by something sudden or completely unexpected. A bit like a deer in headlights, except more like a pigeon that has been bopped by a relatively harmless but still unignorable low-energy projectile.

Breakdown:

We begin with the noun 鳩 (hato), “pigeon,” “dove,” marked as the subject of a clause by the particle が (ga). Next comes compound noun 豆鉄砲 (mamedeppou), literally “bean iron cannon” – in other words, a pea-shooter. This in turn is marked by the particle を (wo) as the object of the verb 食う (kuu), literally “to eat,” or in this case the more figurative “to receive” (with a negative connotation), and finally the noun よう (you, rhymes with “go”), “appearance,” “similar to,” etc. The final-position noun means that this saying is actually a noun phrase rather than a complete sentence.

Notes:

The phrase can be rearranged into 豆鉄砲を食った鳩のよう (mamedeppou wo kutta hato no you), “like a pigeon that got shot by a pea-shooter.” It can also be contracted to just 鳩に豆鉄砲 (hato ni mamedeppou), “a pea-shooter to a pigeon.” But in every case, make sure you don’t lose the 豆. The pigeon is extremely startled, but it’s not dead.

Viewers of certain genres of anime will be familiar with くらう (kurau), which has a similar meaning to 食う’s usage in this saying. A number of my sources stress that no, the pigeon is not eating the pea-shooter, nor its ammunition (at least not yet). Rather, it “ate” a hit, and is still recovering from the shock.

Example sentence:

プレゼントを開けて豆鉄砲を食った鳩のように目を丸くして無言になった男の子は、ガッカリしているのか、それとも喜びのあまり声がでないのか、僕にはしばらく分からなかった。

(Purezento wo akete mamedeppou wo kutta hato no you ni me wo maruku shite mugon ni natta otoko no ko wa, gakkari shiteiru no ka, sore tomo yorokobi no amari koe ga denai no ka, boku ni wa shibaraku wakaranakatta.)

[After opening the present, the boy was left wordless and wide-eyed as a stunned pigeon. Whether he was disappointed, or unable to speak in overwhelming joy, was not clear until later.]