1911 Encyclopædia Britannica/Doldrums

DOLDRUMS (a slang term, dol = dull; cf. tantrum), the region of calms near the equator where the trade-winds die away, a region of constant precipitation in which the weather is close, hot, vaporous and extremely dispiriting. In the old days of sailing vessels, a becalmed ship sometimes lay helpless for weeks. A letter from this region saying “we are in the doldrums” (“in the dumps”) seems to have been regarded as written from “The Doldrums,” which thus became the name of this undesirable locality.