When you want to post something on someone's Facebook
wall, but the fact that the information concerns someone else who might see deems the information un-postable on the wall, therefore unwallable.
Person 1: Dude that is HILARIOUS I need to put that on Vicky's wall
Person 2: Dude it's unwallable, Sasha's friends with her on Facebook
Person 1: Dude you're so right, close call!