1. Not exhibiting or characterized by careful thought
   "unthinking innocence involves knowledge of good and evil, but the knowledge is not deep enough";
   - 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
   "The unthinking student struggled to keep up with the class discussion";
   - lumpish, lumpen

1. In a thoughtless or careless manner, without consideration
   "He spoke unthinking, offending his colleague with his casual remark";
   - thoughtlessly, unthinkingly

See also: daft [Brit, informal], dumb, foolish, imbecile, imbecilic, inconsiderate, stupid, thoughtless