The Collaborative International Dictionary of English v.0.48:

Unleash \Un*leash"\, v. t. [1st pref. un- + leash.]
   To free from a leash, or as from a leash; to let go; to
   release; as, to unleash dogs.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unleash
    v 1: release or vent; "unleash one's anger"
    2: release from a leash; "unleash the dogs in the park"
    3: turn loose or free from restraint; "let loose mines"; "Loose
       terrible plagues upon humanity" [syn: unleash, let loose,
       loose]

Moby Thesaurus II by Grady Ward, 1.0:

58 Moby Thesaurus words for "unleash":
   cast off, detach, diffuse, disengage, disjoin, disperse, doff,
   ease, ease off, free, let go, let up, liberate, loose, loosen,
   relax, release, remove, scatter, slack, slack off, slacken,
   take off, unbar, unbind, unbolt, unbridle, unbuckle, unbutton,
   uncage, unchain, unclasp, undo, unfasten, unfetter, ungag, unglue,
   unhandcuff, unhobble, unlace, unlash, unlatch, unlock, unloose,
   unloosen, unmanacle, unmuzzle, unpen, unpinion, unshackle, unstick,
   unstrap, untether, untie, untruss, unyoke, vent, weigh anchor