stay until the last dog is hung: stay until the very end of an event

Stay until the last dog is hung is at least as old as the Civil War and was very common in the postwar West. It meant to stay until the very end. In recent times, it usually refers to overstaying one's welcome at a party. The origins of the phrase are obscure, but some etymologists speculate that it may refer to an actual activity. In medieval and early modern Europe, animals were sometimes hanged for "crimes" such as killing livestock or biting someone. Both dogs and cats were publicly hanged. What dog is hanging was a seventeenth-century way of asking what all the excitement was about, and Give a dog a bad name and hang him has been a familiar maxim since at least the 1700s. Whether dog hangings happened in nineteenth-century America is unclear. Another possibility is that the dogs referred to were human. Rustlers, gun slingers, and other such "dirty dogs" were routinely strung up from the nearest tree in frontier days, often without benefit of finicky refinements such as trials.