WordWeb Online

1. Lacking the thinking capacity characteristic of a conscious being
   "the shrieking of the mindless wind"
    
2. Requiring little mental effort
   "mindless tasks"
    
3. Not mindful or attentive
   "while thus mindless of his steps he stumbled"
   - unmindful, forgetful
    
4. Showing a lack of intelligence or thought; stupid and silly
   - asinine, fatuous, inane, vacuous
    
5. Not marked by the use of reason
   "mindless violence"
   - reasonless, senseless

See also: amnesiac, amnesic, daft, dumb, foolish, heedfulness, meaningless, mindfulness, mindlessly, mindlessness, nonintellectual, nonmeaningful, stupid, unreasonable