Sped Student Late?

Listen here, buddy. You see, "sped" is a derogatory term for someone in special education, and "tardy" means late. The joke plays on the similarity between "tardy" and the offensive term. It's like, haha, because you're the funniest guy in the world, right? Do you even know the history of the word 'retarded'? I bet you think it's just a funny word. Come on, dude, some words aren't cool to toss around.