A Mexican Halloween is a sexual act. It occurs when a man is having sex with a woman from behind and upon ejaculation, he opens up a can of chili con carne he had hidden earlier, and throws the contents of the can onto the woman's back.
Note: The chili con carne can sometimes be replaced with salsa sauce.
1: Why are you late?
2: I'm sorry, my boyfriend Mexican Halloweened me and I had to clean my back. Some even got on my bed.