My year 12 jersey:
The prevailing joke at the time was that McDonalds hadn't officially fired me, so I was "McJobless" since I was still technically on their system as an employee. I needed a name for my jersey so that stuck.
I started using it for all my user accounts. At some stage I made a joke about how I would get an alt called "McJob" when I finally got work, and then one of you funny forgeters provided.