High Five-jacking refers to the act of hijacking someone's high five. This happens when two people are about to high five, but a third person raise his/her hand and gives one of them a high five before the initial one is performed, thereby hijacking it.
The action is also quite similar to a high five leech.
Kris: I'm so stoked now, Hennick! Give me a high five!
Magnus raises his hand faster than light and gives Kris a high five while Kris is trying to high five Hennick (the High Five-jacking).
Kris: S**t, we were high five-jacked!
Hennick: That was AWESOME!!!
Kris: Yeah, I'm even more stoked now!