The Collaborative International Dictionary of English v.0.48:

Unguarded \Unguarded\
   See guarded.

WordNet (r) 3.0 (2006):

unguarded
    adj 1: lacking protection or a guard; "an unguarded gate"; "his
           unguarded queen was open to attack"
    2: displaying or feeling no wariness; "an unguarded remark"

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "unguarded":
   arbitrary, artless, asleep, bare-handed, bluff, blunt,
   born yesterday, broad, brusque, candid, capricious, careless,
   childlike, confiding, culpably negligent, defenseless, derelict,
   direct, downright, explicit, exposed, forthright, frank,
   frankhearted, free, free-speaking, free-spoken, free-tongued,
   genuine, goofing off, guardless, guileless, hasty, heart-to-heart,
   heedless, helpless, imprudent, inadvertent, inattentive,
   incautious, inconsiderate, indiscreet, ingenu, ingenuous, innocent,
   inobservant, involuntary, laissez-faire, lax, loose, naive,
   napping, neglectful, neglecting, negligent, nodding,
   noninterfering, nonrestrictive, off-guard, open, openhearted,
   outspoken, overly permissive, permissive, plain, plain-spoken,
   procrastinating, reasonless, relaxed, remiss, round, scamping,
   simple, simplehearted, simpleminded, sincere, single-hearted,
   single-minded, skimping, slack, sleeping, slighting, slurring,
   straight, straight-out, straightforward, thoughtless, transparent,
   trustful, trusting, unalert, unarmed, unarmored, unattended,
   uncalculating, uncautious, unchecked, uncircumspect, unconscious,
   unconstrained, uncovered, undefended, undesigned, unequivocal,
   unfortified, ungarrisoned, unguided, unintended, unintentional,
   unmeant, unmeditated, unplanned, unpremeditated, unprepared,
   unprompted, unprotected, unpurposed, unready, unreasoning,
   unreflecting, unreserved, unrestrained, unrigorous, unscreened,
   unsheltered, unshielded, unsophisticated, unsuspecting,
   unsuspicious, unthinking, unthought-of, unthoughtful, unvigilant,
   unwarned, unwary, unwatched, unwatchful, unwise, unwitting,
   vulnerable, weaponless