The New Student's Reference Work/Doldrums

Dol′drums. The term doldrums probably is a latinized form of the word dull. In the early days of ocean-navigation it was applied to the regions near the equator where for months at a time there are intense heat and a dead calm, except for occasional squalls and light, very variable winds. These areas were dangerous to sailing-ships, especially until their exact location was known. Food and drink might give out before the vessel escaped from the calm. The area of doldrums moves north in July and south in January. In the it moves between the equator and 10° north, in the  between 15° south and 15° north. At any time the area covers about 5° of latitude. It is supposed that from these areas start both the tropical hurricanes and those currents of the ocean that flow from the equator to the poles.

A person who is in a depressed or sullen mood, with some tendency to gusts of temper, is said to be in the doldrums.