Hahahahhahaha I love the picture of the dog! With this picture
are you sure you are asking about nagging and not wagging?
Anyways "to nag" simply means to constantly find fault with what another is doing and letting them know continuously.
Another definition "to nag" is when something is causing you continual pain. For example, I have a "nagging headache"
To nag also can be when one keeps worrying over an issue.
Hope this helps :)