water-hemp
noun
1Water agrimony, especially Eupatorium cannibinum (now rare ).
2 US any of several North American plants of the genus Amaranthus (family Amaranthaceae), especially A. cannabinus, a tall marsh plant with willow-like leaves and greenish-yellow flowers, and A. tuberculatus, a troublesome agricultural weed.
Origin
Mid 16th century; earliest use found in John Withals (d. c1555), lexicographer.