The word is derived from a unit in Warcraft III, the gyrocopter (flying machine). Your immediate response when someone masses gyros (to be funny, ruin the game, mess around or whatever) is to rofl for ten straight minutes until the copters of doom and destruction have pinpricked 1 of your 20 farms to death with their uber godly bombs.

Now mostly used in Blizzard circles as a leeter version of rofl.
'...and this guy said I could reduce lag by up to 100% by hitting alt-F4. I did and the game closed on me. This is griefing, admins plz ban'
'ROFLCOPTER!!!!!!!!!!'