When a man is sexually fisting a woman (ex. Shoving his whole fist in her vagina) he uses it to lift her off the ground

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.