Used prenominally to mean 'such a stupid/absurd (something)' — for example, 'such a ridiculous reason.' Often used conversationally with surprise or criticism.
そんな馬鹿な言い訳は通用しないよ。
Such a ridiculous excuse won't work.
そんな馬鹿な意見を真剣に受け取る必要はない。
There's no need to take such a foolish opinion seriously.
An interjection expressing disbelief or 'no way!' — commonly used alone as 'そんな馬鹿な!'.
そんな馬鹿な！合格したなんて信じられない。
No way! I can't believe I passed.
『彼が犯人だって？』『そんな馬鹿な！』
'He is the culprit?' 'No way!'
そんな馬鹿なと思ったが、証拠は確かだった。
I thought 'that can't be,' but the evidence was solid.