I'm pretty sure Nassim Nicholas Taleb wasn't the source, but I first heard the term from him.
His explanation was pretty good, roughly: you can say FU while you are still on the phone with someone instead of after you hang up.
In Soul of a New Machine, there is talk about the formation of Data General in 1969. The attorney advising the founders suggested that they set aside $1m each as part of any deal for the purpose of FU money. The usage may predate even that.
I don't believe so. That term has been used for years prior to 2000 in competitive offshore fishing.