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

• outermost
• outerwear
• outface
• outfall
• outfield
• outfielder
• outfight
• outfit
• outfitted
• outfitter
• outfitting
• outflank
• outflow
• outflowing
• outfox
• outgas
• outgeneral
• outgo
• outgoer
• outgoing
• outgrow
• outgrowth
• outguess
• outgun
• outhaul