Doldrums. Interestingly, this is a plural word that has no singular…anymore. Once upon a time, there was indeed a singular version, and a doldrum was a “dull person.” (Dol is a variation of dull.) Over time, however, that meaning disappeared, and was replaced entirely with a similar-but-different meaning: “low spirits; a depressed or lethargic state of mind.”
So here’s the part I find fascinating. That noun came into being around 1800. I first heard it, however, in a nautical sense–the doldrums being literally without wind in your sails, something which happens especially around the equator. I always assuming that the nautical phrase came first and then was applied metaphorically to the mental state. But nope. The nautical sense came about in the 1820s, because of the state of mind the sailors were in when their ships were becalmed.
And here’s the next funny step. Sailors were so frequently in the doldroms in that particular geographic location (around the equator) that people thought that they meant the doldrums were the place. And so by the 1840s, “the doldrums” are what the calm, windless areas around the equator also came to be called. Who knew?