The Collaborative International Dictionary of English v.0.48:

Unthoughtful \Unthoughtful\
   See thoughtful.

WordNet (r) 3.0 (2006):

unthoughtful
    adj 1: not exhibiting or characterized by careful thought [syn:
           unreflective, unthinking, unthoughtful]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "unthoughtful":
   arbitrary, capricious, careless, disregardful, forgetful, heedless,
   ill-advised, ill-considered, ill-contrived, ill-devised,
   ill-gauged, ill-judged, impolitic, imprudent, inadvertent,
   inadvisable, inconsiderate, indiscreet, inexpedient, injudicious,
   insensate, irrational, mindless, misadvised, misguided, myopic,
   reasonless, reckless, respectless, senseless, shortsighted,
   thoughtless, unaccommodating, unadvised, uncalculating,
   uncomplaisant, unconsidered, uncooperative, undiscerning,
   unforeseeing, unguarded, unheedful, unhelpful, unmindful,
   unobliging, unreasonable, unreasoning, unreflecting, unreflective,
   unseeing, unsensible, unsound, unthinking, unwise, witless