Bookshelf Babe
A hot/attractive women you see, that you do you want have sex with, but cannot, since you already have a girlfriend and you are not scum enough to cheat on her. She is basically like a book that you don't read, that sits on the book shelf collecting dust.
Janice worked in the office right next to me, she was so beautiful that I often daydreamed about her in the middle of work, however I already have a girlfriend and don't want to risk an office romance either, she'll always be that bookshelf babe to me.