Nah it's just me nigga is a phrase spoken by the puppet character Chef Pee Pee from the YouTube channel _SuperMarioLogan_'s video SML Movie: Bowser Junior's Doll!. After the video has been uploaded, many remixes of the phrase began to appear on YouTube and Twitter.

Origins

On September 11th 2017, the 16th anniversary of the September 11 attacks, YouTube channel SuperMarioLogan uploaded a video called SML Movie: Bowser Junior's Doll!, In the video, Bowser Junior or mostly Junior, played by a plush toy, get's grounded for throwing water balloons onto Chef Pee Pee, who is cooking at the time. As a punishment, Junior's father, Bowser, grounds him and forces him to stay at his room. After Junior got grounded, Chef Pee Pee came into his room and plays multiple pranks at Junior. One of the examples being him disguised as Juniors favourite toy, Thomas the Tank Engine:

Chef Pee Pee: enters Juniors room with a large Thomas the tank engine over his head Oh! Hey Junior! It's me! Thomas! What's the matter buddy?
Junior: stops crying Thomas?!
Chef Pee Pee: pulls over Thomas Nah it's just me nigga. leaves Junior's room and slams his door