The Collaborative International Dictionary of English v.0.48:

Candock \Can"dock\n. [Prob. fr. can + dock (the plant). Cf. G.
   kannenkraut horsetail, lit. "canweed."] (Bot.)
   A plant or weed that grows in rivers; a species of
   Equisetum; also, the yellow frog lily (Nuphar luteum).
   [1913 Webster]