expression, ichidan verb
open one's mouth wide; gape
Describes literally opening the mouth wide, often in surprise, yawning, or to eat something large.
彼は驚いて大口を開けた。
He opened his mouth wide in surprise.
ライオンが大口を開けてあくびをした。
The lion yawned with its mouth wide open.
Compound of 大口 (おおぐち, 'big mouth') + を + 開ける (あける, 'to open').