A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.

Variation of "Ya feel me?" the derivative of "You know what I am saying?" a form of "Do you understand what I am trying to explain?"