In a text message or online, the abbreviation "LMAO" stands for "laughing my ass off." It is generally used in response to a joke or a situation that is found to be particularly humorous.
Several such abbreviations exist for expressing laughter via text message or online chats using Internet slang or chatspeak. "LOL" is an abbreviation that means "laughing out loud." "ROFL" stands for "rolling on the floor laughing," and it is sometimes combined with "LMAO" to produce "ROFLMAO," meaning "rolling on the floor laughing my ass off." There is also the related abbreviations "ROTF," for "rolling on the floor."