Source : Webster's Revised Unabridged Dictionary (1913)

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} ({S. hieracifolius}).
       --Loudon.