The Collaborative International Dictionary of English v.0.48:

Mexican \Mex"i*can\, prop. a.
   Of or pertaining to Mexico or its people. -- n. A native or
   inhabitant of Mexico.
   [1913 Webster]

   Mexican poppy prop. n. (Bot.), a tropical American herb of
      the Poppy family (Argemone Mexicana) with much the look
      of a thistle, but having large yellow or white blossoms.


   Mexican tea prop. n. (Bot.), an aromatic kind of pigweed
      from tropical America (Chenopodium ambrosioides).
      [1913 Webster]

WordNet (r) 3.0 (2006):

Mexican poppy
    n 1: annual herb with prickly stems and large yellow flowers;
         southern United States to West Indies and Mexico [syn:
         Mexican poppy, Argemone mexicana]