After you wake up from sleeping and you lay in bed with complete silence relaxing. The only activity that can be done between is a morning pee. you must return to your bed and lay down for it to be fat sleep.
Joe woke up and layed in his bed fat sleeping for two hours, adding to his sloth like ways.
Joe: I fat slept for like 3 hours
Mike: You're a real slothman and a FOF to boot.
When you wake up, usually after travelling in a vehicle for an extended ammount of time with a rock hard member desperately searching for someone to touch it. Not to be confused with the travel fat where the person is fully awake during the fatting up.
Man I wish I wish had a crowbar to smash this muthafucken sleepy fat down!