rock-dumb

derogatory rare slang

(rare, slang, derogatory) Extremely stupid.

Japanese Meaning

非常に愚か、不合理な行動をとるほど馬鹿であること / 極端に知能が低い、ばかげた行動を示す

That rock-dumb idea cost us the contract and left everyone speechless.

あのとてつもなく愚かなアイデアのせいで契約を失い、みんな呆然とした。

Sense（1）

(rare, slang, derogatory) Extremely stupid.

Nothing like a rock-dumb cartoon to speak for non-thinking people!