The Collaborative International Dictionary of English v.0.48:

Unnerve \Un*nerve"\, v. t. [1st pref. un- + nerve.]
   To deprive of nerve, force, or strength; to weaken; to
   enfeeble; as, to unnerve the arm.
   [1913 Webster]

         Unequal match'd, . . .
         The unnerved father falls.               --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unnerve
    v 1: disturb the composure of [syn: faze, unnerve,
         enervate, unsettle]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "unnerve":
   abate, affright, agitate, alarm, attenuate, blunt, castrate, cramp,
   cripple, crush, curdle the blood, damp, dampen, dash, deaden,
   debilitate, demasculinize, demoralize, desex, desexualize,
   devitalize, discomfit, disconcert, dismay, disquiet, dull,
   effeminize, emasculate, enervate, enfeeble, etiolate, eviscerate,
   exhaust, extenuate, faze, fluster, fright, frighten, funk, gruel,
   horripilate, intimidate, knock down, lay low, make one tremble,
   mitigate, overcome, perturb, prostrate, psych out,
   raise apprehensions, rattle, reduce, ruffle, sap, scare, shake,
   shake up, soften up, spook, stagger, startle, sterilize, stun,
   stupefy, unbrace, undermine, undo, unman, unsettle, unstrengthen,
   unstring, upset, weaken