Slack is hard to define, but the SubGenius must have Slack! Slack is the main goal of the Church of the SubGenius. Bobbies and Pinks don't have any Slack. Only true Yetinsyny have Slack. Slack CAN be money, Slack CAN be Sex, but SLACK IS "Bob"! The Slack that can be described is not true Slack.
Unlike those Christians and their tirades about Original Sin, The Church teaches us that all of us, humans and SubGenii alike, are born with Original Slack. As part of their mission to suppress and subjugate the SubGenii, The Conspiracy starts stealing your Slack from the day you are born. They have most of the Slack you SHOULD have had by now, despite not knowing what it is. Paradoxically, stealing Slack becomes easier the less of it there is around.
To cover for all the Slack they have stolen, the Conspiracy have developed techniques of promoting False Slack as if it were the real thing. False slack is what most people use to cope with modern living: planned recreation; approved pharmaceuticals; pre-packaged hobbies; prostitutes; support groups; manufactured lifestyle one can buy off the shelf with no thought nor participation at all. False slack merely winds the tentacles of the CON deeper into your brain.
False slack provides a nice illusion, but the intent is solely as a release valve; it allows you to stay "sane" or "recharge" enough to return to work and keep making profits for other people, just like days off that you "earn" or are "given" at your job. It always costs money to obtain and fades away shortly after the money is gone.
Often a SubGenius will experience what a normal might view as 'failure' or a 'setback', like losing a job. To a SubGenius this is just a dose of Involuntary Slack being foisted upon them by "Bob".
Slack in media
This video from the USA television network's old Night Flight programming explains Slack:
Here's another video showing how even the undead can enjoy some Slack: