The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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: quassia, bitterwood, Quassia amara]