VERB (2)

1. try to forget; put out of one's memory or knowledge;

2. discard something previously learnt, like an old habit;

The Collaborative International Dictionary of English v.0.48:

Unlearn \Un*learn"\, v. t. [1st pref. un- + learn.]
   [1913 Webster]
   1. To forget, as what has been learned; to lose from memory;
      also, to learn the contrary of.
      [1913 Webster]

            I had learned nothing right; I had to unlearn
            everything.                           --Milner.
      [1913 Webster]

   2. To fail to learn. [Obs.] --Dr. H. More.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unlearn
    v 1: try to forget; put out of one's memory or knowledge
    2: discard something previously learnt, like an old habit