Belowdecks

Full definition of belowdecks

Adverb

1. (nautical) Below the main deck of a ship.
• 2003, Edward T. O'Donnell, Ship Ablaze: The Tragedy of the Steamboat General Slocum, page 54Lastly, Lundberg went belowdecks to inspect the boat's bulkheads and hull—for what, it was not clear, since he had absolutely no knowledge of steamboat design.