開いた口が塞がらない
(Aita kuchi ga fusagaranai; “The open mouth does not close”)

Definition:

To be in a state of utter shock, especially in response to someone’s condition, attitude, words, or behavior. To be left speechless, with one’s mouth hanging open. Agape (but not in the Greek sense).

Breakdown:

We begin with the verb 開く (aku), “to open [a thing],” “to have one’s [mouth/eyes] opened,” appearing in a form that is technically past tense, but which functions in this case as a prenominal. This allows it to attach to and modify the noun 口 (kuchi), “mouth,” which is marked by the particle が (ga) as the subject of a sentence. The predicate for this subject consists entirely of the verb 塞ぐ (fusagu), “to block,” “to shut,” in imperfective form and taking the negative ending ない (nai) in conclusive form, which wraps the sentence up nicely.

Notes:

While the cause of the shock can be something good, bad, or extreme-but-neutral, it seems to be most commonly used in response to something negative, especially bad behavior.

This comes to us from a line in a jōruri rendition of the famous 『忠臣蔵』 (Chuushingura) story – the tale of the 47 ronin.

Example sentence:

「何度も同じ嘘をついて、バレてるのに、今回もまたつくのかよ？開いた口が塞がらないな！」

(“Nando mo onaji uso wo tsuite, bareteru no ni, konkai mo mata tsuku no ka yo? Aita kuchi ga fusagaranai na!”)

[“You’ve told the same lie over and over again, and everyone knows it’s a lie, but here you go again? I’m just dumbfounded!”]