A large expression of computer expressions (meaning a reaction or exclamation to something funny) put together into one word. "ROFL" meaning "Rolling on the floor laughing", "LOL" meaning "Laugh out loud", "OMG MY BBQ" meaning something I don't think anyone knows, and "LMFAO" meaning "laughing my fucking ass off".
Frank (Typing): HAHAHAA DID YOU WATCH THAT FUNNY VIDEO?
Bob (Typing): Pfft YEAH!!! I WAS LIKE ROFLLOLOMGMYBBQLMFAO