1.) Short for, "Just kidding rolling on (the) floor laughing my ass off", usually expressed in good humor.
2.) A combination of jk, rofl, and lmao.
3.) An alternative to jk, rofl, lmao or any combination of the three.
patient: How long do I have left to live?
doctor: Two weeks.
doctor: Jkroflmao! You are fine.