Literally, it means “the man with no brain”

We normally use this phrase to describe people not wise or rational enough, who might have done something stupid and silly or made a bad decision based on emotions rather than reason.

@aliasnv @Hammer Thanks for your comments.

It means "笨蛋".But "没长脑袋的家伙" is more serious.Teacher may use this phrase to chew out students.

“没长脑子”吧？就是傻的意思。