She has a nagging pain in her knee that keeps her from resting well.

nagging 是动词 nag 的现在分词形式作形容词用。nag 本意是"不断唠叨、烦扰"，引申为"持续不断的、挥之不去的"。 nagging pain 指那种持续不断、隐隐作痛的疼痛感，虽然不剧烈但让人难以忽视。