It could be a jealous rival from another village. This rival sees Naruto as a threat and decides to play a mean prank on him by giving him a harsh pinch on the ass. This could then lead to a rivalry - based story where Naruto has to prove himself against this rival not just in combat but also in terms of dignity.
One possible plot could be that it's a prank gone wrong. Maybe one of his friends, like Kiba, was trying to play a silly joke on Naruto during a training session. Naruto then overreacts and it leads to a comical chase around the training ground as he tries to get revenge.
It could be Sasuke. Maybe as part of some strange training or prank, Naruto reshapes Sasuke's ass. It would be a really unexpected and funny scenario considering Sasuke's usually serious demeanor.
In a fanfic, Naruto might give up on his friends if he were to be constantly let down by them. For instance, if his friends repeatedly failed to show up when he needed them the most, or if they made fun of his dreams in a really cruel way. Also, if Naruto was somehow brainwashed or influenced by an evil force, he could lose his connection with his friends. He might be made to believe that his friends were a hindrance to his power - up or his destiny, and thus give up on them.
Naruto might meet his future children. He would be able to see what kind of people they are and what their dreams and aspirations are. It would be an emotional moment for him, seeing his family in the future.
Another candidate could be Kakashi. As Naruto's sensei, he is in charge of Naruto's training and development. If Naruto continuously disobeys orders or fails to progress as he should, Kakashi might use this unorthodox method of spanking to get Naruto back on track.
His sensei, Iruka. Iruka has always been like a father figure to Naruto. He would be there with a big smile, giving Naruto words of wisdom and pride. His presence would mean a great deal to Naruto on such an important day.
Well, it's really hard to say exactly what the plot would be with such a strange title. Maybe it could be about Naruto getting into some really wacky adventure where he discovers something odd about himself, like a new power related to that strange phrase. But this is just pure speculation.
Another example could be if the fanfic was about a group of characters trying to solve a complex mystery in a particular way. Percy Jackson shows up and uses his typical 'go - it - alone' style that is so characteristic of him in the original books. This might undermine the group dynamic that the fanfic writer was trying to establish, and make the whole mystery - solving process seem too easy or forced, thus ruining the fanfic.
Another possibility is that a powerful shinobi, like Kakashi, gives the order. Kakashi has been Naruto's mentor and he might order Naruto to stay for his own good. Maybe he sees some danger that Naruto is not aware of and wants him to stay in the relative safety of Konoha.
People within the gay community who are interested in stories about young gay men's experiences and flirtations might be interested.