IPA: /ˈbækˌsiːt/
KK: /ˈbækˌsit/
A seat located at the rear of a vehicle, typically used by passengers.
The children sat in the backseat during the long car ride.
Backseat → It is formed from "back" (from Old English "bæc", meaning the rear part) and "seat" (from Old English "sæte", meaning a place to sit). The word "backseat" refers to a seat located at the rear part of a vehicle.
Think of the 'back' as the rear part and 'seat' as a place to sit — a backseat is simply a seat located at the back.