To make a copy of data as a security measure. It's important to back up your files regularly to prevent data loss.
To support or reinforce an argument, claim, or person. She asked her colleague to back up her proposal during the meeting.
To move backward, especially in a vehicle. He had to back up the car to let the truck pass.
To accumulate in a way that causes a delay or obstruction. Traffic began to back up on the highway due to the accident.