stimulus (n.)

plural stimuli, 1680s, originally as a medical term, "something that goads a lazy organ" (often the male member), from a modern use of Latin stimulus "a goad, a pointed stick," figuratively "a sting, a pang; incitement, spur," from PIE *sti- "point, prick, pierce" (see stick (v.)). General sense of "something that excites or arouses the mind or spirit" is from 1791. Psychological sense is first recorded 1894.