Simply, a bag or rag or magazine or anything container outside a bathroom that you shoot your load into when your done jacking off.
term used to describe someone who's only worth is being there to absorb your masturbation
2. "Alright, jack bag, you don't have to yell"
In other words, a stuck up person (commonly referred to as an asshole) who does stupid things to get attention
Friend #2: "Yeah I have one you can borrow. I slept in this one when I was deployed to Afghanistan"
Friend #1: "FUCK NO!!!! I aint sleepin' in no Jack bag}!!!!"
Now on his own, he reaches into his "jackbag" to revisit his memories with her to begin his self-love. Joe has moved Betty and Trixie out of his "jackbag" to make room for Sally.
2. I couldn't sleep last night so I reached deep into my "jackbag" and had a splooge-fest that knocked me out.
"Altman, I can't believe you did that in that bathroom, that was a real jackbag move of you!"