The Collaborative International Dictionary of English v.0.48:

Masterwort \Mas"ter*wort`\, n. (Bot.)
   (a) A tall and coarse European umbelliferous plant
       (Peucedanum Ostruthium, formerly Imperatoria).
   (b) The Astrantia major, a European umbelliferous plant
       with a showy colored involucre.
   (c) Improperly, the cow parsnip (Heracleum lanatum).
       [1913 Webster]

WordNet (r) 3.0 (2006):

masterwort
    n 1: any plant of the genus Astrantia [syn: astrantia,
         masterwort]