From Encyclopedia Dramatica
OMGWTFBBQ originates from Something Awful; this word was originally used as a surname for one of the writers there. It was meant to mock the use of acronyms on games such as Starcraft and programs like AIM. It was popularized, ironically, when people began to say it as rejection of people who overused these phrases.
It also stands for:
OMG - My house is on FIRE!!!!11!!!ONE
WTF - There are Cows where my kitchen used to be!!!!1!!!
BBQ - The cows are also on FIRE!111!!1!!!one!