this actually means: oh my god what the fuck barbeque
the OMGWTF actually have some relevance to the phrase however the word BBQ doesn't. But BBQ does have a purpose because by adding BBQ on to any phrase it helps to better express the shock, anger or any other emotion.
Nerd 1: I just got a 50 kill streak on CoD4
Nerd 2: Prove it
(Nerd 1 shows Nerd 2 his kill streak record)
Nerd 2:OMGWTFBBQ as if!