The largest and most important sail on a ship, attached to the mainmast (the main vertical pole), which provides most of the propelling force.
A compound of 'main' (chief, primary) and 'sail.' This nautical term has been used since at least the 14th century as sailing ships became the dominant form of ocean transportation.
The mainsail was so crucial to ships that sailors could steer and control a vessel using just this one sail, which is why learning to handle it was the first thing apprentice sailors mastered—it was literally the foundation of sailing.
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