The Collaborative International Dictionary of English v.0.48:

Cowbane \Cow"bane`\ (kou"b[=a]n`), n. (Bot.)
   A poisonous umbelliferous plant; in England, the Cicuta
   virosa; in the United States, the Cicuta maculata and the
   Archemora rigida. See Water hemlock.
   [1913 Webster]