The single most positive affirmation between friends, colleagues, what have you. Said affirmation carries more weight than even those of normal knucks, secret hand shakes, high fives, etc. An Upknuck is preformed when two, or more, people bump fists with metacarpals facing toward other participant and phalanges facing up towards the sky. "Turn that pound upside down."
Rules to Upknuck:
1. You don't deny an Upknuck at any point one is offered. Said denial of Upknuck can result in said Upknucker refusing any further Upknuck related offers to said Upknuckee with Upknucker being completely justified.
2. You don't blow up an Upknuck. Ever.
Dude 1: "Just got a rim job from Sally!"
Bro 2: "Upknucks, brooooo!"