Similar in meaning to "WFT?" or Mr. T's "What you talkin bout?" Used to express shock, disbelief, or a failure to comprehend and often used in internet settings such as message boards, chats, etc..
(Example of use in chat)
JJmg453: Dude, me and joseph got totally wasted last night and fobbed your cousin's schnob!
(Example of phrase used in conversation)
"Hey, I just bought a new car!"
"Dude, I thought you were broke!"
"Yeah, my girlfriend's dad just like, gave me a wad of cash out of the blue and told me to go buy myself a Nissan."
"What fuck you say!?"