impetus (n.) Look up impetus at
early 15c., impetous "rapid movement, rush;" 1640s, with modern spelling, "force with which a body moves, driving force," from Latin impetus "an attack, assault; rapid motion; an impulse; violence, vigor, force;" figuratively "ardor, passion," from impetere "to attack," from assimilated form of in- "into, in, on, upon" (see in- (2)) + petere "aim for, rush at" (see petition (n.)).