The feeling of a little car sickness mixed with the sun beating through the window making you really warm as you are a passenger a car traveling to a destination that you do not want to go to. This feeling typically presents itself during Saturday errands with your Mom when you are in the backseat of the car and would rather be dead than running errands with your Mom.
Feeling brain-dead, stupid or slow. Where any complex thought makes your brain hurt.
Derived from the feelings of slowness one experiences the day after heavy consumption of alcohol, typically on a Friday. The phrase can be used at any time of slowness though, without alcohol being involved.
Sylvestor: Hey dude, you doing any revision today?
Gerald: Nah man, I've got the Saturday Morgans.
Binge eating and drinking on a Saturday as a form of revelry. Rewarding oneself on a Saturday with food, drink, and entertainment without a care for diet.