The Collaborative International Dictionary of English v.0.48:

Unman \Un*man"\, v. t. [1st pref. un- + man.]
   [1913 Webster]
   1. To deprive of the distinctive qualities of a human being,
      as reason, or the like. [R.] --South.
      [1913 Webster]

   2. To emasculate; to deprive of virility.
      [1913 Webster]

   3. To deprive of the courage and fortitude of a man; to break
      or subdue the manly spirit in; to cause to despond; to
      dishearten; to make womanish.
      [1913 Webster]

            Let's not unman each other.           --Byron.
      [1913 Webster]

   4. To deprive of men; as, to unman a ship.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unman
    v 1: cause to lose one's nerve; "an unmanning experience"