The Collaborative International Dictionary of English v.0.48:

Causeless \Cause"less\, adv.
   Without cause or reason.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Causeless \Cause"less\, a.
   1. Self-originating; uncreated.
   [1913 Webster]

   2. Without just or sufficient reason; groundless.
      [1913 Webster]

            My fears are causeless and ungrounded. --Denham.
      [1913 Webster]

WordNet (r) 3.0 (2006):

causeless
    adj 1: having no justifying cause or reason; "a senseless,
           causeless murder"; "a causeless war that never had an
           aim"; "an apparently arbitrary and reasonless change"
           [syn: causeless, reasonless]
    2: having 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" [syn: causeless, fortuitous, uncaused]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "causeless":
   accidental, adventitious, aimless, aleatory, casual, chance,
   chancy, contingent, designless, destinal, dicey, driftless,
   dysteleological, fatal, fatidic, fluky, fortuitous, haphazard,
   iffy, incidental, indeterminate, indiscriminate, inexplicable,
   mindless, promiscuous, purposeless, random, risky, stochastic,
   stray, unaccountable, uncaused, undetermined, undirected,
   unexpected, unforeseeable, unforeseen, unlooked-for, unmotivated,
   unpredictable