rumble seat

noun

1. An extra passenger seat or row of seats in a carriage or automobile, typically folding away into a rear storage compartment.

   “I suspected I wouldn't care an awful lot for his companionship, so I put him and another one in the rumble seat, and with the other two in with me I found the trail back to the Parkway and turned south.”

   “Vito does a lot of his own felting, dealing direct with Indians who are apt to show up here at all hours with a rumble seat full of beaver hides […].”

2. A jump seat.