The doldrums is a colloquial expression derived from historical maritime usage, in which it refers to those parts of the Atlantic Ocean and the Pacific Ocean affected by the Intertropical Convergence Zone, a low-pressure area around the equator where the prevailing winds are calm. The low pressure is caused by the centrifugal force from the rotation movement of the Earth which is most important at the equator, which makes the air rise and travel north and south high in the atmosphere, until it subsides again in the horse latitudes. Some of that air returns to the doldrums through the trade winds. This process can lead to light or variable winds and more severe weather, in the form of squalls, thunderstorms and hurricanes. The doldrums are also noted for calm periods when the winds disappear altogether, trapping sail-powered boats for periods of days or weeks. The term appears to have arisen in the 18th century – when cross-Equator sailing voyages became more common.
Colloquially, the "doldrums" are a state of inactivity, mild depression, listlessness or stagnation.
- All in a hot and copper sky,
- The bloody Sun, at noon,
- 'Right up above the mast did stand,
- No bigger than the Moon.
- Day after day, day after day,
- We stuck, no breath no motion;
- As idle as a painted ship
- Upon a painted ocean.
The colloquial expression was used in Norton Juster's children's book Phantom Tollbooth, in which a place called "The Doldrums" is inhabited by the Lethargarians who follow the same list of activities every day, which mostly include lounging around and sleeping, just like the colloquial meaning of the word. 
- "Doldrums" on the Merriam-Webster Dictionary site
- Dictionary.com, based on the Random House Dictionary, Random House, Inc., 2011.
- The Phantom Tollbooth by Norton Juster