spoiler (n.)

1530s, "one who robs or plunders," agent noun from spoil (v.). Meaning "one who mars another's chance at victory" is attested from 1950 in U.S. politics, perhaps from boxing. Aeronautics sense is from 1928, because the flap thwarts the "lift" on the plane; transferred to structures serving a similar purpose on speedboats (1957) and motor vehicles (1963). Meaning "information about the plot of a movie, etc., which might 'spoil' it for one who has not seen it" is attested by 1982.