1. Scorched, burned; dried out by heat or excessive exposure to sunlight: "We saw this vast desert all adust." 2. Affected by excess bodily heat; hence, displaying signs of dehydration.

Adust came from Latin adustus, the past participle of adurere, "to set fire to; a verb formed from the Latin prefix ad-, "to, a direction toward" 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; all of 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 (heated) black bile in particular was believed to be a source of melancholy (a tendency to be gloomy and depressed).