Defintion 1: to inadvertently hit someone with your backpack due to:
1. Forgetting you are wearing it.
2. Misjudging the depth a backpack adds to your frame
Defintion 2: Backwacker: a person who backwacks
Person 1: Hey, how did you get that scrape on your face?
Person 2: I was backwacked on the subway ride to work today.