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in the doldrums

1. lacking activity or progress When the economy is in the doldrums, every business feels the effects.
2. feeling sad and lacking the energy to do anything He's been in the doldrums since his girlfriend left for college.
Usage notes: the opposite is out of the doldrums: Maybe inviting him to dinner will get him out of the doldrums. -- http://idioms.thefreedictionary.com/

The doldrums is a colloquial expression derived from historical maritime usage for 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 heat 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.

When the winds are gone the sea actually has no swells, on a clear day the color of the sky is reflected in the water. At night the same effect, with no clouds or moon, gives one the effect of floating in space.

The term appears to have arisen in the 18th century (when cross-Equator sailing voyages became more common). It is derived from dold (an archaic term meaning "stupid") and -rum(s), a noun suffix found in such words as "tantrum". --Wiki

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, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.

-from The Rime of the Ancient Mariner by Samuel Taylor Coleridge