means A railway carriage fitted with beds or berths so passengers can sleep during an overnight journey.

from A plain English compound: 'sleeper,' one who sleeps, joined to 'car,' the railway sense of which comes from Latin 'carrus,' a wheeled wagon, by way of Old French. Such cars appeared as rail travel stretched across long distances in the 19th century — most famously the Pullman sleeping cars in America — and the descriptive name simply stuck to the room you snored in while the world rolled past.