The Collaborative International Dictionary of English v.0.48:

Unbridled \Un*bri"dled\, a. [Pref. un- not + bridled.]
   Loosed from the bridle, or as from the bridle; hence,
   unrestrained; licentious; violent; as, unbridled passions.
   "Unbridled boldness." --B. Jonson.
   [1913 Webster]

         Lands deluged by unbridled floods.       --Wordsworth.
   [1913 Webster] -- Un*bri"dled*ness, n. --Abp. Leighton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unbridled
    adj 1: not restrained or controlled; "unbridled rage"; "an
           unchecked temper"; "ungoverned rage" [syn: unbridled,
           unchecked, uncurbed, ungoverned]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "unbridled":
   a bit much, abandoned, bibulous, boundless, crapulent, crapulous,
   debauched, disobedient, disorderly, dissipated, dissolute,
   egregious, enormous, exaggerated, excessive, exorbitant,
   extravagant, extreme, fabulous, fancy, fast, free, gallant, gay,
   gigantic, gluttonous, go-go, headstrong, heady, high, hyperbolic,
   hypertrophied, immoderate, incontinent, indulgent, inordinate,
   insubordinate, intemperate, irrepressible, irresponsible, lawless,
   lax, licentious, loose, monstrous, mutinous, nonrestrictive,
   obstreperous, out of bounds, out of control, out of hand,
   out of sight, outrageous, overbig, overdeveloped, overgreat,
   overgrown, overindulgent, overindulging, overlarge, overmuch,
   overweening, permissive, prodigal, profligate, rakehell,
   rakehellish, rakehelly, rakish, rampant, reinless, revolutionary,
   riotous, self-indulgent, self-willed, steep, stiff, swinish,
   terrorist, too much, unaccountable, unchecked, uncoerced,
   uncompelled, unconscionable, unconstrained, uncontrolled, uncurbed,
   undisciplined, undue, unforced, unfrugal, ungoverned, uninhibited,
   unlimited, unmastered, unmeasured, unmuzzled, unreasonable,
   unreined, unrepressed, unreserved, unrestrained, unrestrictive,
   unruly, unsubdued, unsuppressed, unthrifty, wanton, wild, wildcat,
   willful