A Jack Pack is pornographic material that is snuck into a jail or prison that is wrapped in plastic for repeated use. The possessor of the jack pack can lick the back of the plastic wrap and stick it to the shower walls while the prisoner masturbates. The possessor can either use it for themselves or they can rent it out to other prisoners for items from the commissary.
Bill: Man, I've been locked up for 14 years and I haven't gotten pussy since. I really could use a shower and a jack pack.
Tom: Hey there guy, I've got a jack pack; you can have it for the night for a can of tuna.