This word came from a misunderstanding between my Wife and Myself. One day while putting the groceries away, I asked her where the milk was. She replied "it's in the bag"! There were at least 10 bags and I said "Which Bag"? She came around the corner and said "What did you call me"? I said "nothing, I asked "which bag""! We both laughed because she thought I had called her a "Bitch-Bag".
The friend who comes along with you to various activities, but decides not to engage in the activity. As a result, he or she is stuck keeping everybody's backpacks, purse, etc. safe from harm.
Similar to the word Bag Boy, except you endlessly made fun of the Bag Bitch
We all had an awesome time at Great America except for John, who pussied out of every ride and was stuck being our Bag Bitch