Does anyone can explain an etymology of this word?
The phrase was first borrowed wholesale from the French homme de guerre to refer to a soldier.
Look what I found when hitting the French translation for man-of-war:
"Le mot est d'origine anglaise, contraction de Man-of-War (littéralement Homme de Guerre)"
I guess it went full circle then!
Further down the article refers to a merchant ship as a man-of-trade.
c.1390, "a soldier," from man (n.) + war. Meaning "vessel equipped for warfare" is from 1484. Man in the sense of "a ship" is attested from 1473 in comb. forms (e.g. merchantman). The sea creature known as the Portuguese man-of-war (1707) is so called for its sail-like crest.