The Collaborative International Dictionary of English v.0.48:

Bistort \Bis"tort\, n. [L. bis + tortus, p. p. of torquere to
   twist: cf. F. bistorte.] (Bot.)
   An herbaceous plant of the genus Polygonum, section
   Bistorta; snakeweed; adderwort. Its root is used in
   medicine as an astringent.
   [1913 Webster]