/rag"weed'/ , n.
any of the composite plants of the genus Ambrosia, the airborne pollen of which is the most prevalent cause of autumnal hay fever, as the common North American species, A. trifida (great ragweed or giant ragweed) and A. artemisiifolia.
[ 1650-60; RAG 1 + WEED 1 , so called from its ragged appearance ]