Avoid this nut at all costs.
If someone asks you, "Do you want a Kopfnuss?" you should not say, "Yes please, I like nuts." A Kopfnuss is not a nut you can put on top of your salad or eat as an energizing afternoon snack. A Kopfnuss is a closed-fisted punch to the head. And when knuckles strike your skull, the result is excruciating pain.
Back in the olden days, teachers used to employ the Kopfnuss as a form of punishment. Fortunately, it has since been replaced with more or less humane methods like detention or extra homework. These days, when the term pops up in school, then it is used figuratively to mean a task that is so challenging, you feel like your head is going to crack open. Some desperate students may prefer the old-fashioned Kopfnuss instead.