The Collaborative International Dictionary of English v.0.48:

Lousewort \Louse"wort`\, n. (Bot.)
   Any species of Pedicularis, a genus of perennial herbs. It
   was said to make sheep that fed on it lousy.
   [1913 Webster]

   Yellow lousewort, a plant of the genus Rhinanthus.
      [1913 Webster]