doldrums

[kopend]

where or what are the doldrums

[heathfield]

The overhead sun in the tropics warms the air which then rises and moves outwards towards the poles. This is how the Trade Winds are created. But in the area where the air is simply rising upward, it's flat calm, there's no wind, and a sailing vessel could be stuck there for weeks on end. This region of calm is known as the Doldrums, and extends about 5 degrees north and south of the equator.

[Quizmonster]

Used figuratively, as in "He's in the doldrums at the moment", it means he's unhappy or low in spirits. That is, of course, based on the idea outlined in Heathfield's response above...that of inactivity or a sense of getting nowhere.