Source : Webster's Revised Unabridged Dictionary (1913)

Motiveless \Mo"tive*less\, a.
   Destitute of a motive; not incited by a motive. --
   {Mo"tive*less*ness}, n. --G. Eliot.

Source : WordNet®

motiveless
     adj : occurring without motivation or provocation; "motiveless
           malignity"; "unprovoked and dastardly attack"-
           F.D.Roosevelt [syn: {unprovoked}, {wanton}]