when a male has sex with a female that is on her period while the males dick is soft. he then lets the blood dry on his penis and gets erect and has sex with the women again

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.