The Collaborative International Dictionary of English v.0.48:

Water dropwort \Wa"ter drop"wort`\ (Bot.)
   A European poisonous umbelliferous plant (Enanthe
   fistulosa) with large hollow stems and finely divided
   leaves.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water dropwort
    n 1: European poisonous herb having tuberous roots, yellow juice
         that stains the skin, yellow flowers and foliage resembling
         celery; all parts extremely poisonous [syn: water
         dropwort, hemlock water dropwort, Oenanthe crocata]