stinging nettle

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A dioecious perennial Eurasian herb (Urtica dioica) having stinging hairs, inconspicuous greenish flowers, and coarsely toothed leaves.
2. n. A perennial herb (Cnidoscolus stimulosus) of the southeast United States, having stinging hairs and fragrant white flowers.

Wiktionary

1. n. A nettle; any of the species of genus Urtica that cause painful short-term skin irritation, especially Urtica dioica, originally Eurasian, now naturalized elsewhere.

WordNet 3.0

1. n. perennial Eurasian nettle established in North America having broad coarsely toothed leaves with copious stinging hairs