Part of Speech: Noun

Definition: a tall upright post, spar, or other structure on a ship or boat, in sailing vessels carrying a sail or sails.

Source: Webster Dictionary

Found in “My Boy Willie” by Anonymous, Stanza 9

“From the cabin-boy to the mainmast high/ Ye must mourn in black for my sailor boy”

Mast means a tall upright post but with the word main being added to that it describes that it is the tallest one from all the rest and the main one to picture. I did not know what a mast was to begin with so learning the definition of mainmast helped me picture what the author was trying to express.