I agree with this definition. A lot of factors go into it - how much cash you have in your bank account, how easy it is for you to pick up a new job, etc. Having "FU" money doesn't mean you have to quit your job at the slightest transgression, but rather you are not desperate for the paycheck and can thus stand up for yourself. This in itself is liberating!

At the very beginning of my career I worked in the lab for 36 hours straight trying to finish a project on a tight deadline. I didn't have any "FU" money at that point obviously. If someone asked me to do that today, there would be no way! I wouldn't quit over it necessarily, but I would still be OK financially if I was fired for saying "your deadline is ridiculous!"