noun, noun which may take the genitive case particle 'no'
A derogatory term for a stupid person, literally 'brainless'. Used as a noun or with の to modify another noun.
あんな脳無しの言うことを真に受けるな。
Don't take what that idiot says seriously.
彼は脳無しだから、何を言っても無駄だ。
He's a moron, so it's useless no matter what you say.
馬鹿 is a more common and versatile insult for 'fool' or 'idiot', while 脳無し is more specifically 'brainless' and harsher.
阿呆 is a Kansai-region variant of 'fool', less harsh than 脳無し and often used jokingly.
Compound of 脳 (のう, 'brain') + 無し (なし, 'without'), literally 'brainless'.