ぱかっと

adverb

with one's mouth wide open; agape

Describes a mouth opening suddenly and widely, often from surprise or shock. Onomatopoeic.

See also: ぽかん

• その光こう景けいに、彼かれはぱかっと口くちを開あけて驚おどろいた。

  He was so surprised at the sight that his mouth dropped open.

• 赤あかちゃんがぱかっと大おおきなあくびをした。

  The baby yawned widely with its mouth open.