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?"