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1.
An Extension of the standard ROFL. This literally means: Rolling On The Floor Laughing My Frikin Butt Off. While the speaker isn't always actually laughing, on the floor, or lacking his rear end, this is a good expression used to denote humor in another's words.
1st guy: Dude! I love Waffles!
2nd guy: ROTFLMFBO!
1st guy: yeah!! funneh!
by Doomdooer September 12, 2006
 
2.
Or as an equally chuckle-inducing alternative: rolling on the floor laughing my fuckin balls off. Which always has that effect on me when I see it writ large in web forums. Maybe I'm just childish!
When I discovered what the acronym meant I was ROTFLMFBO for a good 30 seconds or so.
by guitarrzzann November 22, 2006
 
3.
Rolling on the floor laughing my freaking butt off
a term used by young people whilst texting
John:(joke joke joke)
Mary: HAHA rotflmfbo
by Ttinio September 10, 2010