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 independentlife of its own.
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.
When you have to pee in a movie but don’t want to miss anything, and must do the following mental equation: will the amount I have to pee cause enough discomfort to diminish my attention capacity for the remainder of the movie to an extent which is greater than the amount of the movie I would miss by making a mad dash to the restroom.
I saw Satantango last night, wonderful film but by the 5th hour I had to start doing pee calculus.