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