When you become an option. You are not first pick of friends. When your friend starts dating someone and you are put up on a shelf until they have no other people to hang with.
I don't need you anymore, I'm dating somebody. So I'll put you up on the shelf until I have a need for you. You are my shelf friend.
by Pix_n_stones July 15, 2017
Usually pertaining to guys, the friends shelf is when a girl labels you a friend, disallowing you to be anything more then "just friends".
I am on the friends shelf because this girl hangs out with me and we get along great but i will never be more then a friend to her.
by Tongnar July 11, 2006