The Collaborative International Dictionary of English v.0.48:

Unloosen \Un*loos"en\, v. t. [1st pref. un- (intensive) +
   loosen.]
   To loosen; to unloose.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unloosen
    v 1: grant freedom to; free from confinement [syn: free,
         liberate, release, unloose, unloosen, loose]
         [ant: confine, detain]
    2: loosen the ties of; "unloose your sneakers" [syn: unloose,
       unloosen]

Moby Thesaurus II by Grady Ward, 1.0:

45 Moby Thesaurus words for "unloosen":
   cast off, detach, disengage, doff, free, liberate, loose, loosen,
   release, remove, take off, unbar, unbind, unbolt, unbridle,
   unbuckle, unbutton, uncage, unchain, unclasp, undo, unfasten,
   unfetter, ungag, unglue, unhandcuff, unhobble, unlace, unlash,
   unlatch, unleash, unlock, unloose, unmanacle, unmuzzle, unpen,
   unpinion, unshackle, unstick, unstrap, untether, untie, untruss,
   unyoke, weigh anchor