An acronym, standing for: Rolling on the Floor Laughing my Fucking Ass Off out Loud. Usually used when something is the highest level of funny.(Usually used in Texting and/or IM'ing.
(IM)
UrbanDictionary321: My ROFL COPTERgoes SOI SOI SOI.
JOHNNY: Hey Mike, remember when we saw Dumb and Dumber for the 1st time when it was in theaters in 1994?
MIKE: Yeah! When it came to the part when Harry was shitting on the toilet, I was ROTFLMFAOOL!!!