If you are talking about Doc, then it is the nickname for Dr. Dre (the doctor)
If you are talking about The D.O.C., he was a Dallas born, west coast based rapper in the late 80's who worked with Dr. Dre. He made a classic album in No One Can Do It Better but his vocal cords were damaged in a car accident shortly after. Recently, I heard that there is a surgery that The D.O.C. may do to get 70% of his voice back