verb: To let someone down/flake out on someone.
noun: One who commits the aforementioned action.
Similar to when someone does something messed up to you, and you call them a bitch for it.
Bob shnigged on me yesterday when he told my parents I ditched class, even though he did it with me.
Sue is such a shnig: she told me she'd pay me back the money she borrowed, but she still hasn't.