Source : Webster's Revised Unabridged Dictionary (1913)

Quassia \Quas"si*a\, n. [NL. From the name of a negro, Quassy,
   or Quash, who prescribed this article as a specific.]
   The wood of several tropical American trees of the order
   {Simarube[ae]}, as {Quassia amara}, {Picr[ae]na excelsa}, and
   {Simaruba amara}. It is intensely bitter, and is used in
   medicine and sometimes as a substitute for hops in making
   beer.

Source : WordNet®

quassia
     n 1: a bitter compound used as an insecticide and tonic and
          vermifuge; extracted from the wood and bark of trees of
          the genera Quassia and Picrasma
     2: handsome South American shrub or small tree having bright
        scarlet flowers and yielding a valuable fine-grained
        yellowish wood; yields the bitter drug quassia from its
        wood and bark [syn: {bitterwood}, {Quassia amara}]