Thumb

A useful appendage, like a tool, but whereas a tool can be used by anyone, a thumb is dedicated to the use of one person or group. Someone and something that is not only broken, but effectively dead without them. Examples: Op-ed columnists with careers so narrow they would be on food stamps without their special interest groups, third-tier politicians that have jumped off the deep-end with crazy but have been appointed to a position by one better at hiding it, nazis, tankies, actual thumbs.

"Shut the fuck up, you thumb."
or
Self, to own thumb: "You're a thumb, but at least you're useful."