Explanation (spoilers from the webcomic): In the original webcomic, the story has progressed to the point where we've learned that Zombie Man's special power is the ability to heal and regenerate himself, even from the most extreme of injuries, and even if his body has been mostly destroyed. (Which explains how he can disembowel himself just for the purpose of doing a joke.)We also find out that he was one of the earlier creations of Dr. Genus and ever since escaping from the House of Evolution he's been hunting their organization down to try to eradicate it. Eventually he finds Dr. Genus and Armored Gorilla, the only two remaining survivors from after Genos and Saitama destroyed their organization early on in the series, and is shocked to see that they have given up their evil ways and are running a small business together.
Zombie Man you're so funnyseriouswhat are you, my grandpa?I shall doa Zombie GagDr. Genus*clap clap clap*That was funny~!!laughing even if it's not funny... ah, the warmth of a grandparent's love...But I only realized that after I finished drawing it. -_-seriousknow what I'm sayin'Armored GorillaI ain't got no internal organs~ in Japanese, naizou ga nai zo -- you can see the joke. It's actually a pretty old joke.Look, a pervert!This makes zero sense if you haven't read the original webcomic...see the comments for an explanation