Nagging is repetitious behavior in the form of pestering, hectoring, or otherwise continuously urging an individual to complete previously discussed requests or act on advice. The word is derived from the Scandinavian nagga, which means "to gnaw". As expressed by Elizabeth Bernstein, a Wall Street Journal reporter, nagging is "the interaction in which one person repeatedly makes a request, the other person repeatedly ignores it and both become increasingly annoyed".
Most naggers don't know they nag -- they think their nagging helps and is just the repetition of useful tips. But the truth is, it's not up to them to decide if they are nagging or not: a helpful reminder becomes a stinging nag when the person who is being nagged says so.