dumb as a rock

Adjective

(US, simile) Very stupid; dumb as a post.

Japanese Meaning

非常に愚かで知性に欠ける / 極めて馬鹿な

Despite the clear instructions, he remained dumb as a rock and made the same mistake.

明確な指示があったにもかかわらず、彼はとても愚かで同じ間違いを繰り返しました。

Sense（1）

(US, simile) Very stupid; dumb as a post.

（ alternative ）