A real belter of a story, especially one which is funny or surprising. A yarn
may be completely truthful, but it is rather common for a yarn to contain a Furphy or two, i.e. exagerrations for the sake of a better story.
It is important to note that listeners will rarely care whether or not a yarn is truthful, as the priority of a yarn is to entertain as opposed to providing an accurate recount. There is a classic saying in
Australia illustrating this point, "never let the
truth get in the way of a good yarn."