Source : Webster's Revised Unabridged Dictionary (1913)

Bitterweed \Bit"ter*weed`\, n. (Bot.)
   A species of Ambrosia ({A. artemisi[ae]folia}); Roman worm
   wood. --Gray.

Source : WordNet®

bitterweed
     n 1: widespread European weed with spiny tongue-shaped leaves and
          yellow flowers; naturalized in United States [syn: {oxtongue},
           {bristly oxtongue}, {bugloss}, {Picris echioides}]
     2: any of numerous chiefly North American weedy plants
        constituting the genus Ambrosia that produce highly
        allergenic pollen responsible for much hay fever and
        asthma [syn: {ragweed}, {ambrosia}]