Its a racist joke using the fact that sufferers of Dyslexia can have poor reading skills and trouble sounding out words. It basically the "N" word from a "D" word.
Dyslexia means you mix up the order of letters or words. "Ginger" is an anagram (same letters, different order) of another word I won't write out here.
trungkiensave t1_iy8330r wrote
What kind of beverage do Nazis hate?
Juice