。
What do you take me for?
Literal
I [object-を] what [quotative-だと] think-are [emphatic-んだよ].
The question starts with 僕を — unusual object-first order placing the speaker in focus. 何だと思ってる = '[you] think of [me] as what' — the [object]+を+[complement]+だと+思う pattern. A confrontational challenge expressing outrage at perceived disrespect. The んだよ closes with explanatory + emphatic force.