A man with a small head is like a pin without any , very apt to into things beyond his depth 没脑筋的家伙好比一根没有头的针,很容易把腿插进非他力所能及的事情里去。
It s just like in the ancient battles , if the general was killed , the whole army would become like a headless snake , fall into complete disarray and be easily defeated 像以前打仗的时候,如果大将军被打败,军队就像没有头的蛇一样,散乱掉了,随便就可以被打败。