If an injury or some kind of sickness or illness (e.g. People are often kept up at night by all kinds of nagging worries.
We can describe these as nagging doubts, nagging fears and so on. If we are worried or anxious about something and we can’t stop thinking about it, we can say that these thoughts, doubts and worries are nagging (at) us. You are planning on doing it later, but the person asks you again and again. Imagine someone asks you to do something (e.g. The idea of repetition is important with this word. The verb nag can be used to describe someone annoying you by repeatedly asking you something, telling you something, reminding you to do something, complaining about something, and so on.