WordNet sense 1 (not exhibiting or characterized by careful thought):

unreflective, unthoughtful

WordNet sense 2 (showing lack of careful thought):

thoughtless

WordNet sense 3 (without care or thought for others):

uncaring

WordNet sense 4 (lacking regard for the rights or feelings of others):

inconsiderate

WordNet sense 5 (lacking or marked by lack of intellectual acuity):

stupid

WordNet sense 6 (mentally sluggish):

lumpish, lumpen

WordNet sense 7 (in a thoughtless manner):

thoughtlessly, unthinkingly