Properly, the space in the after part of a vessel, under the cabin, but used generally to indicate any part of a vessel having the poorest accommodations and occupied by passengers paying the lowest rate of fare
The section of a passenger ship that provided inexpensive accomodation with no individual cabins
the part of a passenger ship where people who had the cheapest tickets used to travel in the past (steerage (15-21 centuries); because this part of the ship was next to the rudder)