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Longman Dictionary of Contemporary English
back seat
noun
EXAMPLES FROM CORPUS
▪ But when they were introduced, everything upstairs had to take a back seat.
▪ Flustered, she reversed, and the groceries she had bought enroute fell off the back seat and flopped to the floor.
▪ He puts me in the back seat with my coat over my knees.
▪ Many feel reluctant to take a back seat and allow their children to enjoy the special attractiveness of the teens and twenties.
▪ Reno unfolded herself, pushed her bulk out of the car, and peeled her dresses off the back seat.
▪ She and Jack were in the back seat and Fogarty was driving.
▪ She pulled-him out slowly as Miguel grabbed the box and tossed it into the back seat.
▪ So when Andrew started seeing Sarah, the Princess took a back seat.
Douglas Harper's Etymology Dictionary
back seat

also back-seat, 1832, originally of coaches, from back (adj.) + seat (n.). Used figuratively for "less or least prominent position" by 1868. Back-seat driver first attested 1926.

Wiktionary
back seat

n. 1 Any of the seats in the rear of a vehicle. 2 (context informal English) A powerless position.

Wikipedia
Back Seat (of My Jeep)

"Back Seat (of My Jeep)" is a song by LL Cool J, released as a single on June 1, 1993, for Def Jam Recordings, taken from his fifth album 14 Shots to the Dome. It was produced and written by LL Cool J and QD III. The song samples a drum loop from The Detroit Emeralds 1973 song " You're Gettin' a Little Too Smart".

"Back Seat (Of My Jeep)" was the most successful single from the critical and commercially disappointing album; it made it to 42 on the Billboard Hot 100 and 24 on the Hot R&B/Hip-Hop songs. "Back Seat" was packed along with another single, " Pink Cookies In a Plastic Bag Getting Crushed by Buildings", which charted at a lower position (#96 Pop, #34 R&B).

The song was sampled in Monica's 1995 song " Don't Take It Personal (Just One of Dem Days)."

Usage examples of "back seat".

Jam his foot down hard and hope that the surge of power would throw her into the back seatbut that was too chancy.