Open English Wordnet
Adjectives
(s)causeless, reasonlesshaving no justifying cause or reason“a senseless, causeless murder”, “a causeless war that never had an aim”, “an apparently arbitrary and reasonless change”
(s)causeless, fortuitous, uncausedhaving no cause or apparent cause“a causeless miracle”, “fortuitous encounters — strange accidents of fortune”, “we cannot regard artistic invention as … uncaused and unrelated to the times”