From the Super Mario Wiki
The You-Know-What is a compact disc containing unknown content in Super Paper Mario to be returned to Bestovius in the Piccolo sidequest. Nothing else is known about it except it is a collector's item, as revealed by Bestovius after returning it to him.
At one point, Bestovius gives the item to Old Man Watchitt, but he never returns it, despite having been asked several times. When the heroes arrive at his house to collect the Training Machine for Merluvlee, Bestovius asks them to get it back in exchange; because Bestovius refuses to reveal any details about it to them, however he simply tells them to ask Watchitt to give him the "You-Know-What," hence the name given to it as an item. The You-Know-What can be obtained only from Watchitt after obtaining Merlumia's autograph for him. Once Bestovius is given the item, he gives Mario and co. the Training Machine.
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