The Collaborative International Dictionary of English v.0.48:

Addle-brain \Ad"dle-brain`\, Addle-head \Ad"dle-head`\,
Addle-pate \Ad"dle-pate\, n.
   A foolish or dull-witted fellow. [Colloq.]
   [1913 Webster] Addle-headed
   Addle-brained

WordNet (r) 3.0 (2006):

addlehead
    n 1: a person with confused ideas; incapable of serious thought
         [syn: addle-head, addlehead, loon, birdbrain]