Is it ensure, insure, or assure?
1772 (adj.); 1794 (n.), from Latin stimulantem (nominative stimulans), present participle of stimulare (see stimulation).
stimulant stim·u·lant (stĭm'yə-lənt)n. An agent that arouses organic activity, strengthens the action of the heart, increases vitality, and promotes a sense of well-being. adj. Serving as or being a stimulant.