1) Phrase made popular in the song "Baby Got Back" by Sir-Mix-A-Lot in the early/mid 90's. Overall a mad shitty song but good if you are from a small town or are black and grew up in a sterile suburb. Phrase can be used to describe a pseudo "hard"/toughguy event where a smaller dude rolls up on a bigger one and feigns a beatdown using jerky violent motions up in his face.
2) Another common usage of this is when you eat at a shitty vegan joint (Ex: Sprouts in South Lake Tahoe) and have to use the bathroom immediately afterwards. You drop a Kenny Loggins and leave the bathroom, only to discover you didn't really finish and have to run back in. Don't act like you don't know.
1) Yo, you see that fool that got all up in Dave's face and was like "uh! double-up, uh! uh!", fool straight-up served him.
2) Dave ate a sub-par steamed egg burrito and had to uh! double-up, uh! uh! in the bathroom at the gas station down the street.