WordWeb Online

1. Not marked by the use of reason
   "a senseless act";
   - mindless, reasonless
    
2. Unresponsive to stimulation
   "drugged and senseless";
   - insensible, out of it
    
3. Serving no useful purpose; having no excuse for being
   "senseless violence";
   - otiose, pointless, purposeless, superfluous, wasted
    
4. (of especially persons) lacking sense, understanding or judgment
   "The human race is probably best characterized as being that species that has the proclivity to play senseless pranks on one another";
   - nitwitted, soft-witted, witless

See also: stupid, unconscious, unreasonable, worthless

Encyclopedia: Senseless