meaning if an old friend or enemy is not around for you to see, then that just means that he is out of your mind, meaning you are not thinking about him/her/them...they are living their own life and you are living yours...

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.