Acronym for Furiously Beat Off. Not just masturbating, this is when you're going at it like the plane's going down and you have to get off before it crashes into the Atlantic. Also useful when you're on top of a shy girl, or see a super cute picture of cats, pandas, or bears.
Also, something can stand for Furiously Beating Off depending on the context, but honorable men can differ.
"Hey Nick, check out this YouTube video of a baby panda hugging a tiger."
"Excuse me for a minute my friend, I must go and FBO."