Kansai dialect phrase expressing strong disbelief or irritation at what someone just said. Equivalent to standard Japanese 何を言ってるんだよ or 何言ってんだよ.
"I'm quitting my job tomorrow." "What the hell are you saying, all of a sudden!"
What the hell are you talking about? That can't be true.
Kansai dialect phrase from 何を言う (what are you saying) + とんねん (Kansai equivalent of ～ているのだ). The とんねん is a contraction of ておるのや → とるねん → とんねん.