The Collaborative International Dictionary of English v.0.48:

adust \a*dust"\ ([a^]*d[u^]st"), a. [L. adustus, p. p. of
   adurere: cf. F. aduste.]
   1. Inflamed or scorched; dried or darkened by heat. "The
      Libyan air adust." --Milton. --AS
      [1913 Webster]

   2. Sunburnt. [archaic] --AS
      [1913 Webster]

            A tall, thin man, of an adust complexion. --Sir W.
                                                  Scott.
      [1913 Webster]

   3. (Med.) Looking or acting depressed; atrabilious; gloomy.
      [archaic] --AS
      [1913 Webster]

WordNet (r) 3.0 (2006):

adust
    adj 1: dried out by heat or excessive exposure to sunlight; "a
           vast desert all adust"; "land lying baked in the heat";
           "parched soil"; "the earth was scorched and bare";
           "sunbaked salt flats" [syn: adust, baked, parched,
           scorched, sunbaked]
    2: burned brown by the sun; "of an adust complexion"- Sir Walter
       Scott