The Collaborative International Dictionary of English v.0.48:

Addle \Ad"dle\, v. t. & i. [imp. & p. p. Addled; p. pr. & vb.
   n. Addling.]
   To make addle; to grow addle; to muddle; as, he addled his
   brain. "Their eggs were addled." --Cowper.
   [1913 Webster]