"nellie" is actually gay slang for feminine homosexuals. Means the same thing as "Mary", basically. Think of Dom DeLouise shrieking "You sissy Marys!" at the all-male chorus line in "Blazing Saddles.
I can't imagine a rapper wanting to project that particular image. Unless the genre has gotten a lot more inclusive lately...
Well, Dom DeLuise would know about gay terms, that's for sure!