The Doldrums

Doldrums (n.)

by 1803, "low spirits, the blues, the dumps," colloquial, probably from dulled, past participle of dull (v.) in the sense of "make (someone) slow-witted," with ending perhaps patterned on Tantrum.

DEAR Girl, 

From Noise and London City,

I'm here among 

the blithe and witty;

Where young and old, 

from ev'ry clime,

Like adepts, learn to murder Time!

If you've the doldrums or ennui,

Forsake the town and come to me.

from "A Marine Picture" in 

The Spirit of the Public Journals 

for 1802, London, 1803

Transferred sense, in reference to sailing ships, "in a becalmed condition, unable to make headway" is by 1824. 

This was extended in nautical use to parts of The Sea near The Equator that abound in calms, squalls, and light, baffling winds (1848) and the weather characteristic of these parts. 

"Apparently due to a misunderstanding of the phrase 'in The Doldrums', 

The State being taken as a locality" [OED].

also from 1803

Trends of doldrums

Dull (v.)

c. 1200, "to lessen the vigor, activity, or sensitiveness of" (transitive), from dull (adj.). 

Of pointed or edged-things, "make less sharp, render blunt," from late 14c. 

Of colours, glass, etc., "remove the brightness or clearness of," late 14c. 

Intransitive sense of "lose vigour, intensity, or keenness" is from late 14c. 

Related : Dulled; Dulling.

tantrum (n.)

1714, tanterum, originally colloquial, of unknown origin.