The phrase "is there hair in your head" is a colloquial expression that is used to question someone's intelligence or common sense. It is often used in a sarcastic or mocking manner. For example, someone might say "Is there hair in your head?" to someone who has just said or done something foolish.
The phrase "is there hair in your head" is a relatively common one, but it is not clear where it originated. It is possible that it is a corruption of the phrase "have you lost your mind?", which has a similar meaning. Regardless of its origin, the phrase "is there hair in your head" is a useful one for expressing disbelief or exasperation.