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Douglas Harper's Etymology Dictionary
main-mast

"the tallest mast, usually located near the center of the ship," 16c., from main (adj.) + mast (n.1).

Wiktionary
main-mast

n. (alternative spelling of mainmast English)

Usage examples of "main-mast".

Two other men, one in the foremast hounds, the other in the main-mast hounds, were placed, one with instructions to watch for sharks, the other to scan the horizon for approaching vessels.