"LMAO" is a shortening of the sentence "Laughing my ass off" which is very commonly used on the internet. A friendlier version of this could be "LMBO" which stands for "Laughing my butt off".
LMAO is one of many common words used in text messaging, instant messaging, chatting, and on Facebook and Twitter . LMAO is an acronym that stands for Laughing My Ass Off.