The Collaborative International Dictionary of English v.0.48:

Madnep \Mad"nep\, n.
   1. (Bot.) The masterwort (Peucedanum Ostruthium), a tall
      and coarse European umbelliferous plant.
      [1913 Webster]

   2. (Bot.) A biennial weed in Europe and America having large
      pinnate leaves and yellow flowers and a bitter and
      somewhat poisonous root; the ancestor of cultivated
      parsnip; called also wild parsnip.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

madnep
    n 1: biennial weed in Europe and America having large pinnate
         leaves and yellow flowers and a bitter and somewhat
         poisonous root; the ancestor of cultivated parsnip [syn:
         wild parsnip, madnep]