Doldrums

To be in the doldrums is to be miserable or in low spirits and is first attested from 1811. The OED says the origin is obscure but that it was probably slang or dialectical from dullard and indeed gives dullard as an alternative meaning from 1824 onwards. The Doldrums, an area of the Atlantic near the Equator where the trade winds neutralize one another were only so-called from about 1855. Thus, The Doldrums gets its name from a word that was already in existence and not the other way round as is sometimes supposed.