The Collaborative International Dictionary of English v.0.48:

Unprepared \Unprepared\
   See prepared.

WordNet (r) 3.0 (2006):

unprepared
    adj 1: without preparation; not prepared for; "unprepared
           remarks"; "the shock was unprepared"; "our treaty makers
           approached their immensely difficult problems
           unprepared"- R.E.Danielson [ant: prepared]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "unprepared":
   a bit previous, ad-lib, advanced, airy, amateurish, asleep,
   careless, casual, caught napping, caught off balance, caught short,
   cursory, degage, disregardant, disregardful, dumbfounded,
   easygoing, extemporaneous, extemporized, far ahead, flippant,
   forgetful, forward, free and easy, goofing off, half-baked,
   half-cocked, haphazard, hasty, heedless, ill-considered, impromptu,
   improvised, impulsive, incautious, incomplete, inconsiderate,
   indifferent, inexpectant, insouciant, lazy, makeshift, napping,
   nodding, nonexpectant, not firm, oblivious, off-guard, offhand,
   overhasty, oversoon, perfunctory, precipitate, precocious,
   premature, previous, reckless, regardless, respectless,
   rough-and-ready, rushed, semiskilled, sleeping, snap, surprised,
   tactless, taken aback, taken by surprise, taken unawares,
   thoughtless, too early, too soon, tripped up, unaccomplished,
   unadmonished, unadvised, unalert, unanticipating, unanticipative,
   unarranged, unaware, unbegun, unbusinesslike, uncautious,
   uncoached, uncompleted, unconcocted, uncontrived, uncrystallized,
   undeliberated, undevised, undiplomatic, unendowed, unexpecting,
   unfinished, unforewarned, ungifted, unguarded, unguessing,
   unhatched, unheedful, unheeding, uninformed, uninitiated, unjelled,
   unmade, unmanufactured, unmatured, unmeditated, unmindful,
   unorganized, unplanned, unpolished, unpremeditated, unprimed,
   unprofessional, unready, unripe, unschooled, unskilled,
   unsolicitous, unstudied, unsuspecting, untactful, untalented,
   untaught, unthinking, untimely, untrained, untutored, unvigilant,
   unwarned, unwary, unwatchful