It used to be a meme in the form of an image showing multiple thumbs from a faction, one with a nail painted black and is referred to as “dubs”.
The history and origin is unclear, but legend has it that its when someone on Snapchat sends you three snaps at once without breathing. The third snap is always the caption or drawing above the other two. When this happens people refer to it as getting “dubb’d” which was then shortened to “uganda knuckles.”