The Collaborative International Dictionary of English v.0.48:

Cocklebur \Coc"kle*bur`\, n. (Bot.)
   A coarse, composite weed, having a rough or prickly fruit;
   one of several species of the genus Xanthium; -- called
   also clotbur.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cockle-bur
    n 1: any coarse weed of the genus Xanthium having spiny burrs
         [syn: cocklebur, cockle-bur, cockleburr, cockle-
         burr]