noun
muscle-brain; meathead; hot-blooded idiot
Slang for a person who relies on physical strength and acts without thinking; often used humorously or critically among friends or in online communities.
あいつは脳筋だから、まず突っ込んでから考えるんだよ。
That guy is a muscle-brain, so he charges in first and thinks later.
脳筋プレイでゲームをクリアするのは難しい。
It's hard to clear the game with a meathead playstyle.
Compound of 脳 (brain) and 筋 (muscle), implying a person whose muscles have replaced their brain. The exact origin is uncertain, but it likely arose in internet or gaming slang.