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]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "unsettle":
   addle, addle the wits, agitate, ball up, becloud, bedazzle,
   befuddle, bewilder, bother, bug, cloud, confuse, convulse, daze,
   dazzle, derange, disarrange, disarray, discombobulate, discomfit,
   discommode, discompose, disconcert, dishevel, disorder,
   disorganize, disorient, disquiet, distract, disturb, electrify,
   embarrass, embroil, entangle, flummox, flurry, fluster, flutter,
   fog, fuddle, fuss, incommode, jar, jolt, jumble, knock galley-west,
   maze, mist, mix up, moider, muddle, perplex, perturb, pother,
   psych, put out, raise hell, rattle, rock, ruffle, rummage, shake,
   shake up, shock, sicken, spook, stagger, stir, throw,
   throw into confusion, trouble, turn, unhinge, upset