At first I thought Killer Bee only used it as a rap gesture. Later on it was shown as a way to compare and equalize forces between two people. Even later, Naruto was shown doing it with Kurama, to fuse their chakra together.
Does the fist bump have any special meaning? I doubt it originated in Japanese culture or mythology, so where did it come from?
I think there isn't a single meaning, and sometimes it might be a simple brofist, showing respect to one another. But there's also to consider that Bee, in a way, uses fist bumps to read the other person's mind/heart/soul, to feel the other person; he does it with the Raikage.
But for example
When Naruto exchanges fist bump with Kurama (The Nine-tailed fox beast): There happens that the two chakra meld/connect, as a sort of energy transfer. Perhaps it's also more than that, but that's basically to show that we don't simply have brofists in Naruto.
I think I found the chapter but it's not flat-out stated. When Bee first exchanges fist bumps with Naruto, he says he won't train him to control his beast: first because he "made a fool out of his rap", but also because when they exchange the fist bumps,
Bee feels the Dark Naruto inside Naruto himself, through them. So unless Naruto solves this problem, he won't be able to proceed to the next stage, i.e. the training with Bee.
After the first meeting with Bee (when Naruto uses the Erotic Jutsu), Motoi does ask him whether they exchanged fist bumps. I think this is a hint to that, because when Naruto confirms,
Motoi takes him and Yamato to the waterfalls. They ask whether that's the place but Motoi says that that's just the first step: Naruto needs to see the truth first, which is where the Dark Naruto is revealed.
Naruto will help save Motoi from the Squid attack that happens then and Bee trusts Naruto can defeat the Dark one.
Hope that helps.
It's mainly a character trait of Killer Bee, he uses it kind of as a symbol of frienship, greeting or mutual respect; however he also uses this gesture to sense other people's chakra and channel his chakra to the other person.
which can be seen when he trains naruto to control the tailed beast
Bumping fists to naruto is the form of highest respect, in the the anime bumping fists is a way to read someones heart and soul without exchanging fists in a fight, when kurama and naruto bump fists it symbolizes acknowledgement between the two who at one point despised each other it's a brothers in arms bond between them now, bumping fists even in culture and reality symbolize respect and acknowledgement.
I rewatched the first real fight at the falls between Naruto and Sasuke with my daughter for the first time and it was Sasuke who asked Naruto if he knew that the higher shinobi could tell each other what they were feeling/thinking without saying a word just by bumping fists. Original Naruto Season 3 Episode 132.
