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

• priorship
• priory
• Priscoan
• Priscoan aeon
• Priscoan eon
• prise
• prism
• prism spectroscope
• prismatic
• prismatically
• prismatoid
• prismoid
• prison
• prison camp
• prison cell
• prison chaplain
• prison farm
• prison guard
• prison house
• prison term
• prison-breaking
• prisonbreak
• prisoner
• prisoner of war
• prisoner of war camp