expression, noun-modifying (irregular)
jaw-dropping; shocking; surprising
Idiomatic expression describing something so surprising or shocking that one's mouth stays open in astonishment. Often used prenominally to modify a noun.
See also: 開いた口も塞がらぬ
彼の突然の辞職は、あいた口が塞がらぬニュースだった。
His sudden resignation was jaw-dropping news.
その光景を見て、あいた口が塞がらぬ思いだった。
Seeing that sight, I was utterly shocked.
Literally 'the opened mouth does not close', a set phrase expressing astonishment. The exact origin is uncertain, but it is a natural metaphorical extension of the physical reaction to shock.