Eminem uses the nickname 'Slim Shady', as shown here. Why does he do this? The name 'Eminem' is already a stage name (his real name is Marshall Mathers) so he surely he doesn't need another stage name...
3 Answers
Slim Shady was constructed as an alter ego, an alternative personality who could say things Marshall Mathers would not.
Bruce Wayne didn't go out on the streets of Gotham, inflicting violence on the criminals he found… he let Batman do that. Slim Shady was similar - a part to be acted as though in a movie, not the author, but an assumed character - the one who was angry, spoke about violence, drugs, rape, murder.
I can't find why he started this, but his early lyrics don't show the cutting if cruel wit of his work since he devised Slim Shady. It obviously freed up his lyrical style, by dissociating himself [Marshall Mathers] from this public persona who could be more 'gangsta'.
Perhaps after some time, he felt more comfortable being "the rapper Marshal Mathers" & not need to hide behind "the rapper Slim Shady". This could possibly explain why the album that followed The Slim Shady LP was just called The Marshall Mathers LP.
Extracted in part from https://en.wikipedia.org/wiki/Eminem
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3Another great Tetsujin answer :) I remember an article where Eminem essentially says exactly this, and explains how it came about, but I can't locate it now. It's worth noting that many other artists have done similar things --created alter egos to unlock different aspects of their creativity. Beyonce's "Sasha Fierce" is one of the most famous, the part of herself that doesn't suffer from stage fright. Commented Apr 27, 2022 at 12:26
Eminem has said that he uses the name "Slim Shady" because it is a alter ego that he uses to say the things that he would never say in real life. He has also said that the name is inspired by Dr. Dre's character in the film "The Slim Shady LP".
Said above is totally correct and that could be where "shady" comes from as for "slim" I could only assume because he was slim back then.