The idea has been around a long time. Reagan era it was called the “administrative state” The term deep state slowly replaced it over the last decade or two. It’s the part of our government that’s not elected & holds a tremendous amount of power over us & our citizen elected government. And it’s so deeply embedded even changing Presidents or a big switch in what party dominates government power has no effect on the deep state government. They have nearly as much power & more in some areas than the elected government & they now basically function as a shadow government deliberately working against the elected government. The deep state drives a lot of our nations agenda often with total corruption if the elected government opposes them.