dumbfounded; open-mouthed
Describes a state of being so surprised or shocked that one's mouth hangs open. Often used adverbially with と or as a noun with する.
Upon hearing the news, he stood there dumbfounded with his mouth open.
Everyone was open-mouthed at the incredible sight.
Compound of 口 (mouth) and あんぐり (onomatopoeic adverb for opening the mouth wide). The exact origin of あんぐり is uncertain, but it is likely a mimetic word.