Helen Keller and the N-Word

Alright, you dingus, let me break it down. So, you see, Helen Keller was this super famous deaf and blind person. You, on the other hand, are just blind to humor. The joke plays on the fact that she was both deaf and blind, and therefore, she wouldn't know she was signing a racial slur. The humor relies on a pretty messed up assumption that she didn't know better. It is funny because the situation is completely absurd and inappropriate.