The You-Know-What is an item found in Super Paper Mario.
In the series of events ending with the heroes receiving Piccolo, the heroes at one point must get the You-Know-What from Old Man Watchitt. In order to get this, however, the heroes must get the autograph from Merlumina. Watchitt does give it away if you give him Merlumina's autograph, saying that he traded it for a better, more compact design.
The You-Know-What is a disk with unknown data on it. This important item is very similar to the Tape in Paper Mario, the Box in Paper Mario: The Thousand-Year Door and Toadstool's ??? in Super Mario RPG: Legend of the Seven Stars. All four of these items contain or show mysterious things that the player cannot know about.