monsoon n.

1580s, "alternating trade wind of the Indian Ocean," from Dutch monssoen, from Portuguese monçao, from Arabic mawsim "time of year, appropriate season" (for a voyage, pilgrimage, etc.), from wasama "he marked." The Arabic word, picked up by Portuguese sailors in the Indian Ocean, was used for anything that comes round every year (such as a festival), and was extended to the season of the year when the monsoon blows from the southwest (April through October) and the winds were right for voyages to the East Indies. In India, the summer monsoon is much stronger than the winter and was popularly spoken of emphatically as "the monsoon." It also brings heavy rain, hence the meaning "heavy episode of rainfall during the rainy season" (1747). Related: Monsoonal.