The Collaborative International Dictionary of English v.0.48:
Hawkweed \Hawk"weed`\ (-w[=e]d`), n. (Bot.) (a) A plant of the genus Hieracium; -- so called from the ancient belief that birds of prey used its juice to strengthen their vision. (b) A plant of the genus Senecio (Senecio hieracifolius). --Loudon. [1913 Webster]