motiveless
(a.)無動機的
Mo·tive·less, a. Destitute of a motive; not incited by a motive. -- Mo*tive*less*ness, n.
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motiveless
adj : occurring without motivation or provocation; "motiveless
malignity"; "unprovoked and dastardly attack"-
F.D.Roosevelt [syn: unprovoked, wanton]