Definition & Meaning of "shit-for-brains"in English

What is the origin of the term "shit-for-brains" and how is it used?

"Shit-for-brains" is a slang insult that likely emerged in English in the late 20th century, combining "shit" (denoting worthlessness or contempt) with "for brains" to vividly describe someone as extremely stupid or incapable of thinking clearly. The phrase paints a blunt metaphor suggesting the person's mind is full of filth rather than intelligence. It is used in informal speech, online discussions, and casual arguments to express strong frustration, ridicule, or contempt for someone's mental capacity. The term is clearly derogatory and offensive, intended to insult rather than amuse.