The Collaborative International Dictionary of English v.0.48:

Jamestown weed \James"town` weed`\ (Bot.)
   The poisonous thorn apple or stramonium (Datura
   stramonium), a rank weed early noticed at Jamestown,
   Virginia. See Datura.
   [1913 Webster]

   Note: This name is often corrupted into jimson, jimpson, and
         gympsum.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Jamestown weed
    n 1: 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, Jamestown weed, common thorn apple, apple of
         Peru, Datura stramonium]