godan verb (-su), transitive verb
unload; discharge cargo
Refers to taking goods off a vehicle, ship, or other carrier. The compound combines 積む (load) and 下ろす (take down), but the meaning is specifically unloading.
トラックから荷物を積み下ろす。
Unload the cargo from the truck.
港で船からコンテナを積み下ろしている。
They are unloading containers from the ship at the port.