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"

Moby Thesaurus II by Grady Ward, 1.0:

125 Moby Thesaurus words for "unman":
   abase, abate, affright, alarm, attenuate, beat down, bend, blunt,
   break, break down, bring low, bring to terms, browbeat, bulldoze,
   bully, castrate, clamp down on, coerce, compel, conquer, cow,
   cramp, cripple, crush, curdle the blood, damp, dampen, dash, daunt,
   deaden, debilitate, degrade, demasculinize, demoralize, deplete,
   desex, desexualize, despotize, devitalize, disqualify, disquiet,
   domineer, domineer over, drain, dull, effeminize, emasculate,
   enervate, enfeeble, enslave, etiolate, eviscerate, exhaust,
   extenuate, fright, frighten, funk, grind, grind down, gruel,
   henpeck, horripilate, humble, humiliate, impoverish, intimidate,
   keep down, keep under, knock down, lay low, lord it over,
   make one tremble, master, mitigate, oppress, overawe, overbear,
   overcome, overmaster, override, overwhelm, paralyze,
   press heavy on, prostrate, psych out, quell, raise apprehensions,
   rattle, reduce, repress, ride over, ride roughshod over, sap,
   scare, shake, shake up, soften up, spook, stagger, startle,
   sterilize, subdue, subjugate, suppress, terrorize, trample down,
   trample underfoot, trample upon, tread down, tread underfoot,
   tread upon, tyrannize, tyrannize over, unbrace, undermine, undo,
   unfit, unnerve, unstrengthen, unstring, upset, walk all over,
   walk over, weaken, weigh heavy on