WordNet (r) 3.0 (2006):

white thistle
    n 1: any plant of the genus Argemone having large white or
         yellow flowers and prickly leaves and stems and pods;
         chiefly of tropical America [syn: prickly poppy,
         argemone, white thistle, devil's fig]
    2: spiny shrub with silvery-scurfy foliage of alkaline plains of
       southwestern United States and Mexico [syn: quail bush,
       quail brush, white thistle, Atriplex lentiformis]