The final "jack sesh" of the day to easy you into slumber
to jack, jacking, jack off, and the like are all slang for male masturbation
Friend 1: Dude, how many times have you jacked it today?
Friend 2: Three, not including my upcoming slumber-jack
Friend 1: I am so wired, I'll never fall asleep tonight
Friend 2: Bro, I have the perfect solution for you! Treat yourself to a slumber-jack, it never fails me