'belowdecks' originates from the combination of 'below' and 'decks', where 'below' meant 'underneath' and 'decks' referred to the floors of a ship.

'belowdecks' changed from the Old English word 'belo' and the Middle English 'dekke', eventually becoming the modern English word 'belowdecks'.

Initially, it meant 'under the deck of a ship', and this meaning has largely remained the same in modern usage.