A person always null of emotion and personality. They frequently exude boring behavior. They are not the life of the party. The real bathroom signs have no face, no expressions, no personality and you can only tell their sex by their clothing.
Bobby: Have you met Dan's girlfriend yet?
Stella: OMG, I did. She is so freaking boring. How lame?
Bobby: Absolutely, she's a total bathroom sign plain bagel. No personality, she completely blends into the walls.