Noun
(dated) An occupant of a large vehicle, such as a firetruck, who helps steer the vehicle from far in the back.
A person sitting (typically) in the back seat of a car issuing (unsolicited and usually unwanted) instructions to the driver.
(figuratively) An opinionated person who offers (unsolicited and often unwanted) advice on the best way to handle a situation.
Source: en.wiktionary.org