The Collaborative International Dictionary
Hydropiper
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.
Wiktionary
hydropiper
n. A form of knotweed, the water pepper, ''Polygonum hydropiper''