expression, noun-modifying (irregular)
such a stupid ...; what a crazy ...
Used as a prenominal phrase to express disbelief or criticism about something absurd. Often followed by a noun like 話 (story) or こと (thing).
そんなばかな話があるわけない。
There's no way such a stupid story could be true.
彼がそんなばかなことをするなんて信じられない。
I can't believe he would do such a crazy thing.
interjection
no way!; what nonsense!; how can that be?
An interjection expressing strong disbelief, shock, or rejection of an idea. Often used in response to surprising or absurd statements.
「明日から会社が休みだって」「そんなばかな！」
"They say the office is closed from tomorrow." "No way!"
そんなばかな、そんなことあるはずが
What nonsense! That can't possibly be true.
Means 'stupid' or 'ridiculous' as an adjective; そんなばかな adds 'such' for stronger emphasis and is often used as a set phrase.
Also expresses disbelief, but まさか is more like 'no way' or 'it can't be', while そんなばかな is more emphatic and dismissive.
A fixed phrase combining そんな (such) and ばかな (stupid, ridiculous). The expression is a natural collocation used to express disbelief.