Your Mom is a standard punchline to an often sardonic, and always illogical, joke that involves a witting or unwitting person posing a question, answered with a dry, deadpan response: "Your mom."
Examples:
"Who will win the World Series?" Answer: Your mom. "Why did Bush invade Iraq?" Answer: Your mom. "Have you finished the TPS reports for our meeting with Inatrode?" Answer: Your mom.