Wiktionary
alt. 1 A passenger in a car who insists on giving the driver directions. 2 (context idiomatic English) By extension, anybody offering unsolicited or unwelcome advice. n. 1 A passenger in a car who insists on giving the driver directions. 2 (context idiomatic English) By extension, anybody offering unsolicited or unwelcome advice.
WordNet
n. a meddler who insists on giving unwanted advice
Usage examples of "backseat driver".
Mouse is relegated to backseat driver status, stuck in the cave mouth.
Hed expected her to be a backseat driver, but that had turned out to be Austins job.
Needless to say, he was carefully watched by the submersible's usual pilot, who was doing his best not to be a backseat driver.
McCoy continues, climbing over Spock in the skimmer, trying not to bump his head as he settles into his seat of choice, inveterate backseat driver.