In low spirits; feeling dull and drowsy.

The Doldrums is the region of calm winds, centered slightly north of the equator and between the two belts of trade winds, which meet there and neutralize each other. It is widely assumed that the phrase 'in the doldrums' is derived from the name of this region. Actually, it's the other way about. In the 19th century, 'doldrum' was a word meaning 'dullard; a dull or sluggish fellow' and this probably derived from 'dol', meaning 'dull' with its form taken from 'tantrum'. That is, as a tantrum was a fit of petulance and passion, a doldrum was a fit of sloth and dullness, or one who indulged in such.

The term was used to mean 'a general state of low spirits' in the early 19th century. For example, this piece from The Morning Herald, April 1811:

"I am now in the doldrums; but when I get better, I will send [for] you."

In 1824, Lord Byron used the phrase in a nautical context in the verse tale The Island:

"From the bluff head where I watch'd to-day, I saw her in the doldrums; for the wind Was light and baffling." [Note: baffling winds are those which are shifting and variable, making progress under sail impossible.]

'In the doldrums' came to refer specifically to sailing ships that were becalmed and unable to progress.

The region now called the 'The Doldrums' wasn't named until the mid 19th century and the naming came about as the result of a misapprehension. When reports of ships that were becalmed in that equatorial region described them as being 'in the doldrums', it was mistakenly thought that the reports were describing their location rather than their state. The earliest known reference to the region's name in print is Matthew Maury's The physical geography of the sea, 1855:

"The 'equatorial doldrums' is another of these calm places. Besides being a region of calms and baffling winds, it is a region noted for its rains."