Dictionary:

adust

  (ə-dŭst')

1. Burned; scorched.
2. Archaic. Browned by the sun; sunburned.
3. Archaic. Melancholy in appearance or temperament; gloomy.

[Middle English, from Latin adustus, past participle of adūrere, to set fire to : ad-, ad- + ūrere, to burn.]

The adjective has 2 meanings:

Meaning #1: dried out by heat or excessive exposure to sunlight
  Synonyms: baked, parched, scorched, sunbaked

Meaning #2: (archaic) burned brown by the sun