third arm
Somebody who considers themself so tied to someone else that they are only there to do the other's bidding. A slave. By analogy with an actual third arm, with which the owner of the arm could hope to do so much more, but which has no independent life of its own.
Example:
Mary is trying to treat Jimmy like her third arm again. She won't even let him go out for a night with the lads.
Mary is trying to treat Jimmy like her third arm again. She won't even let him go out for a night with the lads.