WARNING: The following contains spoilers for One Piece Chapter #1014 by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English now from Viz Media.
One Piece Chapter #1014 begins where Chapter #1013 left off, with Luffy plummeting from Onigashima towards the sea below after Kaido defeated him. While this is a major upset, another shocker follows close behind: Kaido namedrops one of the most mysterious characters in all of One Piece.
At the start of the chapter, Kaido goes over the fight to himself. Copying Kaido, Luffy figured out how to infuse his body with his Conqueror's Haki, using it like Armament Haki. However, as Kaido puts it, Luffy's method was sloppy, resulting in his defeat. After tossing Luffy overboard, Kaido makes a strange remark, saying Luffy "couldn't be Joy Boy either."
Ever since Joy Boy was first mentioned during the Fishman Island arc, he's been the center of many mysteries and much speculation. Over 1,000 chapters into One Piece, there's still a lot of unknowns regarding Joy Boy -- for a long time the only facts we had came from the account that Robin translated from the Sea Forest Poneglyph.
According to Robin and King Neptune, Joy Boy was someone who existed 800 years ago, during the Void Century, who made a promise to the people of Fishman Island during that time. He swore to bring the fishmen to the surface with the massive ark, Noah, and the power of the ancient weapon, Poseidon, but the plan never came to fruition. However, the people of Fishman Island agreed to keep Noah safe as Joy Boy assured them someone would eventually come in the future to fulfill his end of the bargain.
Robin actually mentions that the Sea Forest Poneglyph is written differently from the previous Poneglyphs she's encountered, reading more like an apology letter from Joy Boy to the Mermaid Princess at that time, who also possessed the Poseidon ability to speak to Sea Kings like Princess Shirahoshi.
Nothing new about Joy Boy would be revealed until fairly recently, during the Kozuki Oden flashback to his travels with the Roger Pirates. After acquiring all four Road Poneglyphs, the Roger Pirates had finally reached the last island in the Grand Line. The treasure they found there was something Joy Boy left behind that oddly makes the entire crew erupt in laughter. The Roger Pirates then decide to give the appropriate name to this unknown island, Laugh Tale, and the rest is history.
What Kaido meant by saying Luffy "couldn't be Joy Boy either" is unclear, but there are two strong theories. Of all the Yonko, including Whitebeard, Kaido is the only one whose age hasn't been revealed yet. There's a strong possibility that Kaido is much older than he lets on, given that we don't even know if he's actually human. He could have been around during Joy Boy's era, 800 years ago. It's likely Joy Boy could stand toe-to-toe with Kaido during that time, but that's purely speculation.
A more likely scenario is that Kaido is aware of Joy Boy's promise to the people of Fishman Island and is keeping an eye on possible candidates to fulfill Joy Boy's end of the bargain. The promise seems to be common knowledge among the Fishmen, as Neptune seemed to know despite not being able to read the Poneglyph. Furthermore, the fact Kaido said "either" means he likely has come across others with potential who failed in the end, like Luffy.
Luffy being the one to fulfill Joy Boy's promise has been a fan theory for years, and given how much the would-be Pirate King loves to prove people wrong, Kaido doubting him only really adds more fuel to the fire in One Piece.