"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".
Friend: Spring is here! I got so excited I wet my plants!
LMAO is one of many commonwords used in text messaging, instant messaging, chatting, and on Facebook and Twitter . LMAO is an acronym that stands for Laughing My Ass Off.