|Origin:||motif, from motif 'moving'; MOTIVE2|
the reason that makes someone do something, especially when this reason is kept hidden:
What do you suppose the killer's motive was?
motive for (doing) something
The police believe the motive for this murder was jealousy.
The motives behind the decision remain obscure.
The violence was clearly prompted by political motives.
It's not the kind of thing he'd do unless he had an ulterior motive (=a reason he kept hidden).
an apparently motiveless killing