I was watching random videos on Youtube and one of them took my attention. I heard a girl saying 'cash me outside'. In fact this video got really popular on Youtube, but I'm not able to make out what she was actually saying there.
Here is one clip I found of it.
Dr. Phil: Did you say "The hoes are laughing"?
DP: So the audience are a bunch of hoes?
(Audience applauds loudly.)
Girl: (To the audience.) Cash me outside, howbow dah.*
Girl: Catch me outside, how bout dat.
DP: Catch you outside? What does that mean?
Girl: What I just said.
Mother: "Catch her outside" means she'll go outside and do what she has to do.
Because of the way she speaks (her accent, dialect) and the loud audience, it's hard to hear exactly how she pronounces it in that instance, but * above is "Catch me outside, how about that" written in standard English. Catch here is informal for meet or find. In her aggressive, hostile tone, catch me outside is an invitation to argue or fight outside. It's meant to provoke the audience.
"Cash me outside" doesn't mean anything.
If what you heard was "catch me outside", then "catch" is being used in a slang way as a synonym for "see" or "meet" - as in "catch you later".
The saying came from a nit-wit on the Dr. Phil show. The girl did indeed say "Cash me outside" as slang for a challenge to go outside and have a fight.
"Cash me outside" means go outside and have a fight. It was first said by Danielle Bregoli AKA cash me outside girl. She said this at the Dr.Phil show. This thing is available in YouTube.
The girl who spoke the phrase is trying to confuse the audience by saying nonsense phrases. She is unable to communicate with Dr Phil or the audience. She is speaking in her made-up code slang words which only she understands. Or she stayed up too late the night before and was having trouble staying awake during the TV interview.
"Cash me outside" is a rap song, which clearly the girl is imitating to sound hostile or aggressive. She will run as fast as she can if anybody took her up on that phony hard behavior.