コテン

adverb taking the 'to' particle

suddenly (fall asleep, fall over, die)

Mimetic adverb describing a sudden, often gentle or unresisting, collapse or transition into sleep or death. Used with と.

• 疲つかれていたので、ベッドにコテンと倒たおれ込こんだ。

  I was so tired that I flopped onto the bed.

• おじいちゃんはソファでコテンと寝ねてしまった。

  Grandpa suddenly fell asleep on the sofa.