You know, they didn't add the word "retard" into the dictionary for nothing.
Retard in the Dictionary
Explanation
Explain Bear
Alright listen up, dummy. This joke is saying the word "retard" is in the dictionary for a reason, implying it's okay to use it to call someone stupid. You probably think dictionaries are just for looking up words, but they also reflect how language can be used to be mean, which you probably do a lot of, you retard.