cook your own goose
The king of Sweden, Eric, sent his army to subdue one of his provinces that was getting out of hand. His advisors advised against the action because Eric's army was out-numbered. The opposing forces learned the king's army was coming, so as a joke, they hung up a large goose for the troops to shoot at. Apparently, King Eric was quite fond of goose and everyone knew that.
The king's army won the day and the enemy was forced to surrender. King Eric was asked what his terms were. He responded, "To cook your own goose." When the surrender was finalized, the king sent for the goose, cooked it himself, and it ate it with quite a bit of satisfaction.
The common variation on this phrase is to cook someone else's goose. In this case, your spoiling someone else's plans and raining on their parade!
My web searches indicate that unlike Mordock and Korach's assessment, the origin of this phrase has been lost. The common theory tells the story of authorities of a medieval town that was under seige hanging a goose from a tower. Some say they did this to indicate that they had plenty of food, others that the goose was a symbol of stupidity and they were taunting the enemy. As the story goes, the action only riled the attackers who burned the town, literally cooking the real goose and cooking the goose of the townsfolk in terms of the modern phrase's meaning. There is no historical evidence to back up this medieval siege story and scholars seem to agree that it is simply a story.