A car's front bumper when used to nudge or hit the car in front of it, either in retaliation or (in stock car racing) to inform the second car's driver that the first intends to pass. Always be considerate when changing lanes in front of other cars, as aggressive drivers may feel inclined to use their chrome horns to signal their displeasure. Some really aggressive driving by Roberts; he's been putting that chrome horn to use today!
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.