1. In a meaningless and purposeless manner
   "these innocent bystanders were senselessly killed" 
2. In an unreasonably senseless manner
   - mindlessly

See also: senseless

• quarter day
• quarter horse
• quarter mile
• quarter note
• quarter plate
• quarter pound
• quarter rest
• quarter round
• quarter section
• quarter sessions
• quarter stock
• quarter-century
• quarter-circle
• quarter-finalist
• quarter-hour
• quarter-tone
• quarterage
• quarterback
• quarterdeck
• quarterfinal
• quartering
• quarterlight
• quarterly
• quartermaster
• quartermaster general