Short for, "Just kidding, all the way". It is used by many of the same
people who say things
like, "See you later, alligator" and "Bye-bye butterfly". It is presumed that the kind of people who say these things
suffered some sort of serious rhyme shortage as children.
"Hey, did you hear? Jerry just
got hit by a car!"
"Oh my god, is he okay?"
"Haha, you fell for it! JK all the way!"