Mugging
Explanation
The Joke
A mugger demands "Gimme all your money!" and the victim asks "Why should I?" The mugger explains he's an extreme-horizon scientist-utilitarian who believes morality should be calculated over vast time spans, and that since the sun will engulf the Earth in 4.5 billion years, all of human history is a brief cycle, and nothing beyond a delimited time horizon matters -- meaning the victim's material possessions are meaningless and the mugger's actions will "autoclave" (sterilize/erase) in time.
The victim responds by taking the mugger's logic and turning it against him: "I took advantage of your philosophical distraction to steal your gun. Gimme your wallet." When the mugger protests, the victim asks "What's your moral relation?" The mugger reveals: "I'm a wallet-losing moral relativist."
The Humor
The comic satirizes philosophical moral reasoning used to justify bad behavior by showing how easily such reasoning can be turned against the person wielding it. The mugger constructs an elaborate philosophical framework involving cosmic time scales and utilitarian calculus to justify a mugging, but the same nihilistic reasoning that supposedly justifies his crime also means he has no grounds to object when the tables are turned. The specific humor is in the ironic reversal -- the victim doesn't defeat the philosophy; instead, they accept the philosophical framework and use its own logic as a weapon. The final self-identification as a "wallet-losing moral relativist" is a perfect punchline that acknowledges the self-defeating nature of using moral relativism to justify theft.
References
The comic references several philosophical concepts: moral relativism (the view that moral judgments are not universal), utilitarianism (calculating the greatest good for the greatest number), and long-termism or extreme time-horizon ethics (evaluating moral actions over cosmic timescales). The mention of the sun engulfing Earth in billions of years references actual astrophysical predictions about the sun's red giant phase. The term "autoclave" is borrowed from medical sterilization, used metaphorically to mean that time will erase all consequences.