Rolling On the Floor Laughing My Fu**ing Brains Out, in short ROFLMFBO, is an adaption of an ''old'' internet accronmym for Rolling On Floor Laughing. The integration of the second part, laughing my f-ing brains out, is probably originated from several animation shows. Like ROFL, ROFLMFBO can be used in a satirical manner.
ROFLMFBO is mostly used to emphasize how extremely funny something really is. Sometimes it is used in reply of the acronym ROFL itself.
<x> Hey dude, did you see this comedian? (link from some site) ROFL!!!