Green’s Dictionary of Slang

(orig. US) anyone who offers unwanted advice to the person who is actually in charge of, or at least performing, the task for which the advice is given; thus backseat driving.

1921		Muskogee Times-Democrat (OK) 14 Dec. 4/1: A back-seat driver is a pest who sits on the rear cushions of a mtor car and tells the driver what to do.
1930		N.Z. Truth 1 May 14/2: What happens when one back-seat driver, male or female, takes the other back-seat driver riding.
1930		San Bernadino Co. Sun (CA) 30 May 20/6: Uncle sam will not function as a beack-seat driver. he will not at oljen time fissuily admonish his nephews to pout of brakes and at anotheer time yel at them to ‘give her the gas’.
1934		Weseen Dict. Amer. Sl.
1940		Ogden Standard-Examiner (UT) 23 Mar. 3/1: And refuse, and stick to it, to drive the car if he persists in being a back-seat driver. Just get out [...] the next time he makes a suggestion about the way you are driving.
1952		J. Jones From Here to Eternity (1998) 194: There’ll be no backseat drivin from the Orderly Room if I take your kitchen.
1965		J. Orton Loot Act I: She always was a back-seat driver.
1970		Post-Crescent (Appleton, WI) 21 Aug. 4/8: About the opnly time a man understand’s a woman’s talk is when she is acting as a backseat driver.
1999		Indep. 11 Aug. 1: Her assertion that she would be ‘a very good back seat driver’ was to plague his early years in office.