Source : Webster's Revised Unabridged Dictionary (1913)

Jamestown weed \James"town` weed`\ (Bot.)
   The poisonous thorn apple or stramonium ({Datura
   stramonium}), a rank weed early noticed at Jamestown,
   Virginia. See {Datura}.

   Note: This name is often corrupted into jimson, jimpson, and
         gympsum.

Source : WordNet®

Jamestown weed
     n : intensely poisonous tall coarse annual tropical weed having
         rank-smelling foliage, large white or violet
         trumpet-shaped flowers and prickly fruits [syn: {jimsonweed},
          {jimson weed}, {common thorn apple}, {apple of Peru}, {Datura
         stramonium}]