from the root words rofl, copter, and bbq, it is when a second party sees a roflcopter, preferably one that goes SWOSWOSWOS, lands on the first party's bbq. the saying is only limited to the second party, and any third party that may come in near contact can say, but not limited to, roflmao, lolzores, and roflmaololzors.
Person 1 'hey ted, the neighbor just had a roflcopter land on his bbq'
Person 2 'ah, a roflcopterbbq'
Person 3 (person outside) 'roflmaololzoers'