The backseat is the row of seats behind the driver in a car. Where the children sit and the arguments happen.
British English usually writes it as two words and American as one. The most useful thing the word has given English is the backseat driver, meaning a passenger who tells the driver what to do, which has become the standard name for anybody offering unwanted instructions about a job that is not theirs. Taking a back seat means deliberately letting somebody else lead.
1nounbackseatThe seat behind the driver in a vehicle.
“The shopping is on the backseat.”
Also accepted in games: back seat
From back + seat.
Etymology from Wiktionary, CC BY-SA