The Collaborative International Dictionary of English v.0.48:

Cobnut \Cob"nut`\, n.
   1. (Com.) A large roundish variety of the cultivated
      hazelnut.
      [1913 Webster]

   2. A game played by children with nuts.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cobnut
    n 1: small nut-bearing tree much grown in Europe [syn: cobnut,
         filbert, Corylus avellana, Corylus avellana grandis]
    2: nut of any of several trees of the genus Corylus [syn:
       hazelnut, filbert, cobnut, cob]