Brett vor'm Kopf - HFF!

The phrase "ein Brett vor dem Kopf haben" ("having a board in front of one's head") is often used to express that someone is obtuse. Originally, the expression had nothing to do with a person's ability to think, but referred to livestock.

They were stubborn oxen that had a board put in front of their eyes - this way it was easier for the farmers to work with the animals. This prevented the oxen from being frightened when the farmer hung the harness around their heads, for example.