The Collaborative International Dictionary of English v.0.48:

Unconstraint \Un`con*straint"\, n.
   Freedom from constraint; ease. --Felton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unconstraint
    n 1: the trait of lacking restraint or control; reckless freedom
         from inhibition or worry; "she danced with abandon" [syn:
         abandon, wantonness, unconstraint]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "unconstraint":
   Bohemianism, abandon, abandonment, accessibility, affability,
   approachability, artlessness, bluffness, bluntness, broadness,
   brusqueness, candidness, candor, casualness, common touch,
   communicativeness, conversableness, cordiality, crapulence,
   crapulency, crapulousness, directness, drunkenness, ease, easiness,
   easygoingness, excess, excessiveness, extravagance, extroversion,
   familiarity, folksiness, forthrightness, frankness, freedom,
   freeness, genuineness, gluttony, graciousness, homeliness,
   homeyness, immoderacy, immoderateness, immoderation, impulsiveness,
   incontinence, indiscipline, indulgence, informality, informalness,
   ingenuousness, inordinacy, inordinateness, instinctiveness,
   intemperance, intemperateness, irregularity, irrepressibility,
   laxness, licentiousness, looseness, naivete, naturalness,
   noncoercion, nonintimidation, offhandedness, openheartedness,
   openness, outgoingness, outspokenness, overdoing, overindulgence,
   permissiveness, plain dealing, plain speaking, plainness,
   plainspokenness, prodigality, relaxedness, riotousness, roundness,
   self-indulgence, simplicity, sincerity, sociability, spontaneity,
   straightforwardness, swinishness, talkativeness, too much,
   too-muchness, unaffectedness, unceremoniousness, unconstrainedness,
   uncontrol, unconventionality, uninhibitedness, unpretentiousness,
   unrepression, unreserve, unreservedness, unrestraint,
   unrestriction, unreticence, unruliness, unsecretiveness,
   unsophistication, unstrictness, unsuppression, untaciturnity,
   wantonness, wildness