adverb, noun-modifying (irregular)
open-mouthed; agape
Describes a state of having one's mouth wide open, often from surprise, shock, or disbelief. Commonly used with する or as an adverb modifying verbs like 見る.
その光景を見て、あんぐりと口を開けた。
Seeing that sight, I opened my mouth wide in amazement.
彼はあんぐりした顔でニュースを聞いていた。
He was listening to the news with his mouth agape.