A contracted form of そんな馬鹿な used prenominally to mean 'such a ridiculous...' or 'such a stupid...'; colloquial.
んな馬鹿な話を信じる人はいない。
No one would believe such a ridiculous story.
んな馬鹿な言い訳で済むわけがない。
There's no way that kind of ridiculous excuse will do.
んな馬鹿な計画を本気で立てるな。
Don't seriously make such a foolish plan.
An interjection (contracted form of そんなバカな) meaning 'No way!', 'That's impossible!', used to express disbelief or surprise.
んな馬鹿な！本当に彼が勝つのか？
No way! Did he really win?
んなバカな、うそだろう。
Come on, that's impossible—no way.
んな馬鹿な、信じられない！
No way — I can't believe it!