Combination of FOCL (Falling off chair laughing), ROFL (Rolling on the floor laughing) and LMAO Laughing my ass off). FOCROFLMAO therefore is "Falls off chair rolling on the floor laughing my ass off".
As a reaction to a funny comment or joke told over the internet.
E.g. Joke Teller: ...so the guy replies, I didn't know your dad was a pharmacist.
Listener: FOCROFLMAO!!!!!
slip of the tongue perhaps,
Those idiots who drive around in a ridiculously raised pick up truck, making a top heavy vehicle even more top heavy and unstable
A:*gah*
B: "Whats the matter"
A: This dam prickup is blinding me.
B: Stupid thing's, as if there lights weren't blinding enough as it is.