Literally, to prick or goad. Hence,

To excite, rouse or animate to action or more vigorous exertion by some pungent motive or by persuasion; as, to stimulate one by the hope of reward, or by the prospect of glory.

In medicine, to excite or increase the action of the moving fibers or organs of an animal body; as, to stimulate a torpid limb; or to stimulate the stomach and bowels.