The Collaborative International Dictionary of English v.0.48:

oxtongue \ox"tongue`\, ox-tongue \ox"-tongue`\, n. (Bot.)
   A widespread European weed (Picris echioides formerly
   Helminthia echioides) with spiny tongue-shaped leaves and
   yellow flowers. The name is applied to several plants, from
   the shape and roughness of their leaves; as, Anchusa
   officinalis, a kind of bugloss, and Helminthia echioides,
   both European herbs. It has been naturalized in the U. S.

   Syn: bristly oxtongue, prickly oxtongue, bitterweed, bugloss.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

oxtongue
    n 1: widespread European weed with spiny tongue-shaped leaves
         and yellow flowers; naturalized in United States [syn:
         oxtongue, bristly oxtongue, bitterweed, bugloss,
         Picris echioides]