The Collaborative International Dictionary of English v.0.48:

Unsettle \Un*set"tle\, v. t. [1st pref. un- + settle.]
   To move or loosen from a settled position or state; to unfix;
   to displace; to disorder; to confuse.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unsettle \Un*set"tle\, v. i.
   To become unsettled or unfixed; to be disordered. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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