The Petticoat affair (also known as the Eaton affair) was an 1830–1831 U.S. scandal involving members of President Andrew Jackson's Cabinet and their wives, who socially ostracized John Eaton, the Secretary of War, and his wife Peggy over disapproval of the circumstances surrounding their marriage. The affair had a radical impact on the Jackson administration and led to the resignation of several cabinet members, and Jackson's subsequent reliance on an informal "Kitchen Cabinet". It aided in Martin Van Buren's rise to the presidency, and was in part responsible for John C. Calhoun's transformation from a national political figure with presidential aspirations into a sectional leader of the slave-holding Southern states.
Margaret "Peggy" O'Neill was the daughter of William O'Neill, owner of The Franklin House, a boardinghouse and bar in Washington, D.C. which was only a short distance from the presidential mansion, making it a popular social center for politicians and military officials. Margaret was well-educated; she studied French and was known for her ability to play the piano. Future Postmaster General, William T. Barry wrote "of a charming little girl... who very frequently plays the piano, and entertains us with agreeable songs." An elderly Margaret reminisced that, "While I was still in pantalets and rolling hoops with other girls I had the attention of men, young and old, enough to turn a girl's head." Her reputation was already under scrutiny because she worked in a bar frequented by men and casually bantered with the boardinghouse clientele.