The Collaborative International Dictionary of English v.0.48:

Colewort \Cole"wort`\, n. [AS. cawlwyrt; cawl cole + wyrt wort.
   Cf. Collards.]
   1. A variety of cabbage in which the leaves never form a
      compact head.
      [1913 Webster]

   2. Any white cabbage before the head has become firm.
      [1913 Webster]

WordNet (r) 3.0 (2006):

colewort
    n 1: a hardy cabbage with coarse curly leaves that do not form a
         head [syn: kale, kail, cole, borecole, colewort,
         Brassica oleracea acephala]