late 14c., "a soldier;" see man (n.) + war (n.). Meaning "armed ship, vessel equipped for warfare" is from late 15c. Man in the sense of "a ship" is attested from late 15c. in combinations (such as merchantman). The sea creature known as the Portuguese man-of-war (1707) is so called for its sail-like crest. The great U.S. thoroughbred race horse was Man o' War (1917-1947).