The Collaborative International Dictionary of English v.0.48:

Opopanax \O*pop"a*nax\, n. [L., fr. Gr. ?; ? vegetable juice +
   pa`nax, pa`nakes, a kind of plant: cf. F. opopanax.]
   The inspissated juice of an umbelliferous plant (the
   Opoponax Chironum), brought from Turkey and the East Indies
   in loose granules, or sometimes in larger masses, of a
   reddish yellow color, with specks of white. It has a strong
   smell and acrid taste, and was formerly used in medicine as
   an emmenagogue and antispasmodic. --Dunglison.
   [1913 Webster]

WordNet (r) 3.0 (2006):

opopanax
    n 1: an odorous gum resin formerly used in medicines