WordNet (r) 3.0 (2006):

loosen up
    v 1: cause to become unblocked; "The medicine unstuffed my nose
         in minutes!" [syn: unstuff, loosen up] [ant: block,
         choke up, lug, stuff]
    2: become less tense, less formal, or less restrained, and
       assume a friendlier manner; "our new colleague relaxed when
       he saw that we were a friendly group" [syn: relax, loosen
       up]
    3: make one's body limber or suppler by stretching, as if to
       prepare for strenuous physical activity [syn: limber up,
       warm up, loosen up]
    4: become less tense, rest, or take one's ease; "He relaxed in
       the hot tub"; "Let's all relax after a hard day's work" [syn:
       relax, loosen up, unbend, unwind, decompress, slow
       down] [ant: tense, tense up]
    5: cause to feel relaxed; "A hot bath always relaxes me" [syn:
       relax, unstrain, unlax, loosen up, unwind, make
       relaxed] [ant: strain, tense, tense up]

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "loosen up":
   amuse, beguile, beguile the time, convulse, delight, divert,
   drown care, ease off, enliven, entertain, exhilarate, fracture one,
   have a ball, have fun, kill, kill time, knock dead, laugh it up,
   live it up, raise a laugh, raise a smile, recreate, refresh,
   regale, relax, slay, solace, tickle, titillate, unbend, unlax,
   unwind, wow