means The gaseous state a liquid or solid takes when it warms up and drifts off as a fine, diffuse cloud — steam, mist, fume.

from From Latin 'vapor,' meaning steam or exhalation, which sailed into English through Old French. The same root breathes through 'evaporate' and 'vapid' — a word that started as 'flat, spiritless,' as if all the lively vapor had risen off and left only the dregs. In the 18th and 19th centuries the plural, 'the vapors,' became a fashionable diagnosis for fits of faintness or melancholy, the idea being that noxious fumes rose from the body to cloud the mind.