1. Basically, a kid posing as a bamf who stands behind his/her crew and is afraid to take real action in a fight.
2. Someone who claims they can kick ass, and they chicken out when someone wants to go.
James: I saw Jason behind those punks fighting after-school today. What a whimp.
Rob: I know, such a jack in the back.
Choosing the easiest and most satisfying way to resolve a situation even if it isn't necessarily the bestway. Refers to the instructions found on a bag of instant light charcoal.
Person 1: I dunno...should I nail my best friend's mom?
Person 2: Just light the bag, man. Just light the bag.