What is the origin of the idiom "thin on top" and when to use it?

The idiom "thin on top" is derived from the observation that individuals who experience hair loss or a receding hairline tend to have less hair specifically on the top of their heads. It is typically used to describe someone who has a noticeable reduction in hair density on the crown or upper portion of their head. This phrase is often used in a descriptive or observational manner and is commonly associated with aging or male pattern baldness.