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]