1. Having no justifying cause or reason
   "a senseless, causeless murder"; "a causeless war that never had an aim"
   - reasonless 
2. Having no cause or apparent cause
   "a causeless miracle"
   - fortuitous, uncaused

See also: unintended, unmotivated

• thirdhand
• thirdly
• thirst
• thirst for knowledge
• thirster
• thirstily
• thirstiness
• thirsty
• thirteen
• thirteenth
• thirties
• thirtieth
• thirty
• Thirty Years' War
• thirty-eight
• thirty-eighth
• thirty-fifth
• thirty-first
• thirty-five
• thirty-four
• thirty-fourth
• thirty-nine
• thirty-ninth
• thirty-one
• thirty-second