One of the mysteries of the English language finally explained.
1A horse-drawn passenger coach that travels by day, especially one that makes a journey in a single day. Compare night coach. Now historical.
2A standard railway passenger carriage, especially as distinguished from a sleeper.
3In extended use: an aeroplane with seating like that in a railway carriage, providing an economical daytime service for a large number of passengers. Frequently attributive Compare "coach". Now rare.
Late 18th century; earliest use found in Gazetteer & New Daily Advertiser. From day + coach.
day coach/ˈdeɪ ˌkəʊtʃ/
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