1) Nautical platform 2) Platform on a ship 
• (n.) The platform about the head of the mainmast in square-rigged vessels.
Main'top` noun (Nautical) The platform about the head of the mainmast in square-rigged vessels. 
The maintop was a wooden platform placed over the head of the mainmast in a ship or brig, serving to extend the rigging and provide standing room to the top-men and their tools.