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}, {devil's fig}] 2: spiny shrub with silvery-scurfy foliage of alkaline plains of southwestern United States and Mexico [syn: {quail bush}, {quail brush}, {Atriplex lentiformis}]