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

See also: senseless

• Synchytrium
• Synchytrium endobioticum
• synclinal
• syncline
• syncopal
• syncopate
• syncopated
• syncopation
• syncopator
• syncope
• syncretic
• syncretical
• syncretise
• syncretism
• syncretist
• syncretistic
• syncretistical
• syncretize
• syncytium
• syndactylism
• syndactyly
• syndesmosis
• syndetic
• syndic
• syndical