Pessimistic Mexican

Okay, listen up, you chucklehead. You clearly don't have the brainpower of a baked bean. This joke is playing on words, see? It takes the word 'Mexican' and twists it to sound like 'Mexican't,' which sounds like 'can't'. It's about a person from Mexico being pessimistic, and that person CANNOT even be positive! Get it? The wordplay creates an absurd image, you simpleton.