Meaning: one who criticizes from the sidelines (definition).


Example: From the other side of the counter, Marge made continual suggestions on how to mix the recipe, in her typical role as back-seat driver.
Origins: From the modern day automobile passenger who freely comments on the driving habits of drivers. (Source.)

Rewrites back seat adviser rumble seat supervisor rear seat moderator coach seat pilot over-the-shoulder editor offsite oversight blind guide 
Discussion: Not sure these are all equivalent rewrites, but they all give a flavor of the metaphor intended.