Sexual act where someone takes it orally and anally/vaginally at the same time. When 2 peoplebookend someone, they are bookshelfing or have bookshelved them.
"Were gonna go bookshelf this girl!" "Oh man me and Phil Bookshelved this slut last night!" "So, when are you gonna let me and my boy bookshelf ya?" "I walked in on my mom being bookshelved by my best friend and my dad!"
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.