The Collaborative International Dictionary of English v.0.48:

Unpremeditated \Unpremeditated\
   See premeditated.

WordNet (r) 3.0 (2006):

unpremeditated
    adj 1: not prepared or planned in advance; "asked an
           unpremeditated question"
    2: not premeditated [ant: premeditated]

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "unpremeditated":
   a bit previous, ad lib, ad-lib, advanced, automatic, casual,
   caught napping, caught off balance, caught short, coincidental,
   extemporaneous, extemporary, extempore, extemporized, far ahead,
   forward, gut, half-baked, half-cocked, haphazard, hasty,
   ill-advised, ill-considered, ill-devised, impromptu, improvised,
   impulsive, inadvertent, indeliberate, involuntary, last-minute,
   makeshift, natural, not firm, off the cuff, offhand, overhasty,
   oversoon, precipitate, precocious, premature, previous, reflex,
   reflexive, rough-and-ready, rushed, snap, spontaneous,
   spur-of-the-moment, surprised, taken aback, taken by surprise,
   taken unawares, too early, too soon, tripped up, unadvised,
   unarranged, unbegun, uncalculated, unconcocted, unconscious,
   unconsidered, uncontrived, uncrystallized, undeliberate,
   undeliberated, undesigned, undevised, unguarded, unguided,
   unhatched, unintended, unintentional, unjelled, unmade,
   unmanufactured, unmatured, unmeant, unmeditated, unorganized,
   unplanned, unprepared, unprimed, unprompted, unpurposed, unready,
   unripe, unstudied, unthinking, unthought, unthought-of, untimely,
   unwilled, unwitting