YMCA, sometimes simply called the Y, is a worldwide youth organization based in Geneva, Switzerland. It was founded on 6 June 1844 by Sir George Williams in London, originally as the Young Men's Christian Association and specializes in promoting healthy living and exercise.
In "Meg's Wedding", Peter tries to go swimming at the YMCA despite having arms that are still in the process of growing back after being torn off in an accident. Despite his insistence that he can participate, he quickly finds out he can only paddle around in circles, and fears being hit by an old retired guy doing laps.