to torment, to sting, to gnaw at (one's conscience)

💡 Nuance & Usage: Often used to describe emotional or psychological torment, like guilt, anxiety, or regret gnawing at someone. Less about physical pain infliction. (後悔や不安などが心を苦しめる様子を表す場合によく使われます。肉体的な苦痛を与える意味合いは薄いです。)