"Naw but, fr fr, those people on Urban dictionary are using it wrong. Trying to invent new slang and sh*t. I thought it was for documenting slang that is already commonly used."
Your response when someone says something so surprising you have to ask if they're for real several times. Can also be used in a sarcastic manner in order to highlight how silly or a statement was.
normal use:
"bro, honestly she beat my ass i dont know how she was so strong"
"fr fr ong????"
sarcastic use:
"bro i keep jacking it to old mia khalifa videos"
"fr fr ong?"
The phrase "fr fr no cap" is a shortened way of saying "For real, for real, no cap". "No cap" is a slang term, meaning "No bullshit", "No shit" etc. and it is commonly used to signal the fact that some kind of information is not false. Therefore, "fr fr no cap" is a phrased commonly used by Gen Z's to signal the same thing: the fact that some kind of information is not false.