The Collaborative International Dictionary of English v.0.48:

Tonka bean \Ton"ka bean`\ [Cf. F. onca, tonka.] (Bot.)
   The seed of a leguminous tree (Dipteryx odorata), native of
   Guiana. It has a peculiarly agreeable smell, and is employed
   in the scenting of snuff. Called also tonquin bean.
   [Written also tonca bean, tonga bean.]
   [1913 Webster]