vapor n.

"low spirits; dull, gloomy state of mind," 1520s, plural of dumpe "a fit of musing," of uncertain origin, possibly from Dutch domp "haze, mist," from Middle Dutch damp "vapor" (see damp (n.)). Compare vapors under vapor.

The application of this term to an affection of the mind is a part of the medical theory which attributed all disorders of the frame to a humour falling on the part affected, and regarded mental disorders especially as produced by a vapour rising from the stomach into the brain. [Hensleigh Wedgwood, "A Dictionary of English Etymology," 1859]

late 14c., from Old French évaporation and directly from Latin evaporationem (nominative evaporatio), noun of action from past participle stem of evaporare "disperse in vapor or steam," from assimilated form of ex "out, out of" (see ex-) + vapor "steam" (see vapor).