Really, Only Fibbing. Looking Moderately Amused Only.
Person A: I'm so tired my eyes hurt
Person B: LOL
Person A: Did you actually LOL?
Person B: No! ROFLMAO
Person A: Are you actually laughing now?
Person B: No.
Person A: You're an idiot.
Internet slang for "Rolling On (the) Floor Laughing My Ass Off". Usually used when "LOL" just isn't enough. There have been cases of which people have added an 'F' to the acronym. Thus being "ROFLMFAO" A.K.A "Rolling on (the) Floor Laughing My F*cking Ass Off".