The phrase "flat head" can refer to a number of different things, depending on context.
In some cases, it may refer to someone or something that is not very bright or intelligent - for example, "He's got such a flat head; he just doesn't get it!" In this sense, the phrase is used as an insult or criticism.
On the other hand, "flat head" can also refer to a type of skull deformation in infants caused by pressure on their heads from sleeping in certain positions. This condition is known medically as positional plagiocephaly and results in a flattened area on one side or the back of the baby's head.
In addition, "flat head" can also refer to something that is not very challenging or difficult - for example, "This job is so easy; it doesn't even require any effort! It's got such a flat head!" In this sense, the phrase is used in a positive way to describe something that is straightforward and uncomplicated.
Overall, the phrase "flat head" can have different meanings depending on context, but generally refers to someone or something that is not very bright or challenging.