There are actually two definitions for the phrase negative retarded.
1.) More retarded than retarded. (The superlative form of retarded)
2.) If used in the proper context, it can be a way to call someone intelligent. This is because the negative of something is usually the opposite, so the negative of retarded is intelligent.
John is negative retarded, he walked into a door like 10 times in row, man!
Christine was called negative retarded today by her friend because she is in smart people math (aka calculus)
Bob didn't know what negative retarded meant, so he must be negative retarded