A person to act on a buffer on a possibly uncomfortable date. A positive spin on the word thirdwheel. This person is a friend of both of the people who would want to hang out and potentially , but are not too familiar with each other yet. So the spare tire is asked to come along to make the date less awkward.
Guy: Aye i want to ask Jane to the movies this weekend but I'm not sure if she'll go, i don really know her that well.