The Collaborative International Dictionary of English v.0.48:

Hydropiper \Hy"dro*pi`per\, n. [NL., fr. Gr. "y`dwr water + L.
   piper a pepper.] (Bot.)
   A species (Polygonum Hydropiper) of knotweed with acrid
   foliage; water pepper; smartweed.
   [1913 Webster]