Someone that is awefully annoying, hideously ugly, or just plain fucking stupid...and deserves to be shanked.
They often do not realize they display these qualities which makes them ever more shankable.
Girl 2: He sounds like a tool.
Girl 1: Shankable?
Girl 2: Definitely!
Girl 1: "That shankable a$$hole over there just grabbed my ass as i walked by."
Girl 2: "Fucking shank 'im"