1
general::
A passenger who gives unwanted and/or unneeded directions to the driver; also, a person who interferes in affairs without having knowledge, responsibility, or authority for doing so. For example, Aunt Mary drives us all crazy with her instructions; she's an incurable backseat driver. This term originated in the United States in the 1920s, when it was first used for a passenger legitimately directing a chauffeur, and it was quickly transferred to figurative use. Also see the synonym MONDAY-MORNING QUARTERBACK and the antonym TAKE A BACK SEAT.
American Heritage Idioms
2
general::
Phrase(s): backseat driver
Fig. an annoying passenger who tells the driver how to drive; someone who tells others how to do things. • I don’t need any backseat driver on this project. • Stop pestering me with all your advice. Nobody likes a backseat driver!
McGrawhill's American Idioms And Phrasal Verbs