ばかだ

is stupid; is foolish

→ ばか
phrase Common #5,965
is stupid; is foolish An informal expression combining ばか (stupid or foolish person) with だ (the plain copula). Depending on context, it can be an insult or a playful remark. The polite form is ばかです.
JA: 彼がばかだなんてとんでもない。
EN: It's absurd to say that he's stupid.

彼は文字どおりばかだ

He is literally an idiot.

ばかだ

You idiot.

— グエムル -漢江の怪物- (2006)

彼がばかだなんてとんでもない

It’s absurd to say that he’s stupid.

彼はいわば学者ばかだ

He's a kind of absent-minded professor.

そんなもの飛びつくのはばかだ

You'd be a fool to jump at something like that.

そんなことを言うなんて君はばかだ

You’re an idiot for saying something like that.

彼は本当にばかだねえ

He’s really an idiot, isn’t he?

お前達二人はどうしようもないばかだ

You two are utterly hopeless idiots.

私の父はいわば学者ばかだ

My father is, so to speak, an absent-minded scholar.

お前は本当にこのなしばかだ

You really are an absolute idiot.

アメリカ人はばかだ

Americans are stupid.

彼はばかだと思っている

I think he's stupid.

その申し出断るなんて君はばかだ

You're foolish to turn down that offer.