Source : Webster's Revised Unabridged Dictionary (1913)

Fust \Fust\, n. [OF. fust cask, F. f?t cask, taste or smell of
   the caak, fustiness, cf. sentir le f?t to taste of the cask.
   See {1st Fust}.]
   A strong, musty smell; mustiness.

Fust \Fust\, v. i.
   To become moldy; to smell ill. [Obs.]