It being related to music is the only correct part of the last definition. A level reached by few, overground is when an underground artist becomes so popular that they can no longer be called underground. I would say that any underground artist who sells 1 mill+ copies of any one of their albums can be considered overground.
Tech N9ne, Tecca Nina, and Aaron Yates are the only overground artists that come to mind. . . they are all the same person if you didn't catch that.
The Urban Dictionary Mug
One side has the word, one side has the definition. Microwave and dishwasher safe. Lotsa space for your liquids.
Buy the mug