In an episode of That Time I Got Reincarnated as a Slime, Rimuru is beginning to talk to his villagers. See episode 3, timecode 17:30.
Right, it took five minutes for all of you to quiet down.
He waits momentarily to give them a chance to respond, then thinks to himself after no response:
They didn't get my best joke?!
Unfortunately, neither do I. It appears to involve the fake mustache, which he discards soon after that. My guess is that this scenario would be familiar to a Japanese audience.
What is the "joke" Rimuru is making? If it is something from Japanese culture, why would he think the evolved goblins and direwolves from this different world would understand it?