A "troll-bump" is the exact same thing as a "fist-bump" (search "fist bump" if you don't know what that is) but used among and between trolls upon successful trolling.
*class was canceled*
Troll 1: Mark, you're late for class! Ask Troll 2!
Mark: Really troll 2?!
Troll 2: Yes! Hurry, run!
*Mark runs off to class*
Troll 1 and Troll 2: *Troll-bump*
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