A rumble seat (American English), dicky seat, dickie seat or dickey seat (British English), also called mother-in-law seat, is an upholstered exterior seat which folds into the rear deck of a two-seat pre-World War II automobile, and seats one or two passengers. When unoccupied, the space under the seat's lid could be used for storing luggage.
The 1865 edition of Webster's An American Dictionary of the English Language defines a rumble as "A boot with a seat above it for servants, behind a carriage." Similar to the rumble on European phaetons was the spider, a small single seat or bench on spindly supports for seating a groom or footman. Before World War I, rumble seats did not always fold into the bodywork.