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Adust comes from Latin adustus, the past participle of "adurere" ("to set fire to"), a verb formed from the Latin prefix ad- and the verb "urere" ("to burn"). It entered the English language in the early 15th century as a medical term related to the four bodily humors - black bile, blood, phlegm, and yellow bile - which were believed at the time to determine a person's health and temperament. "Adust" was used to describe a condition of the humors in which they supposedly became heated or combusted. Adust black bile in particular was believed to be a source of melancholy. The association with melancholy gave rise to an adjectival sense of adust meaning "of a gloomy appearance or disposition," but that sense is now considered archaic.
Origin of adust
Middle English, from Latin adustus, past participle of adurere to set fire to, from ad- + urere to burn — more at ember
First Known Use: 15th century
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