Abbrevation for; "Rolling on floor, laughing my fucking ass off" and somtimes used "Rolling on floor, laughing my fat ass off." It is a mixture of "Rofl" which is "Rolling on floor laughing" and "lmfao" "laughing my fucking/fat ass off."
Bob: "Lmfao I just fucking pwned j00, ROFLMFAO!"
Greg: "Dude, you fucking newb0r, you didn;t fucking pwn anyone but your mom. Dude your gay... hah I just fucking killed you... ROFLMFAO!"