WordWeb Online

1. Not exhibiting or characterized by careful thought
   - unreflective, unthoughtful
    
2. Without care or thought for others
   "the unthinking saying of a great princess on being informed that the people had no bread; 'Let them eat cake'"
   - thoughtless, uncaring
    
3. Mentally sluggish
   - lumpish, lumpen

1. In a thoughtless manner
   - thoughtlessly, unthinkingly

See also: inconsiderate, stupid, thoughtless