LMFAO is a term used when someone is laughing really hard. It can mean 2 different things. Term 1: laughing my fucking ass off. Term 2: laughing my fat ass off.
John: Aww, that little girl just dropped her ice cream I feel so bad...
Kate: LMFAO LOOK SHES CRYING HAHAHHAAHHA
John: It's not funny...
Kate: IT IS!!! HAHHAHAHA