The Collaborative International Dictionary of English v.0.48:

Clotbur \Clot"bur`\, n. [Cf. Clote.]
   1. The burdock. [Prov. Engl.] --Prior.
      [1913 Webster]

   2. Same as Cocklebur.
      [1913 Webster]

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):

clotbur
    n 1: any of several erect biennial herbs of temperate Eurasia
         having stout taproots and producing burs [syn: burdock,
         clotbur]