expression, noun-modifying (irregular)
jaw-dropping; shocking; surprising
Used when something is so surprising or shocking that one's mouth stays open in astonishment. Often modifies nouns like 話, ニュース, or 結果.
See also: あいた口が塞がらぬ
彼の突然の辞職は、開いた口も塞がらぬニュースだった。
His sudden resignation was jaw-dropping news.
あいた口も塞がらぬほど驚いた。
I was so shocked that my mouth hung open.
Variant with が instead of も; essentially the same meaning and usage.
A set phrase literally meaning 'even an open mouth won't close,' used to express extreme surprise or shock.