Origin of (FS) dreadnought is nothing at all.
People just call stuff it whenever they want to.
On a personal basis, I would think of a destroyer-sized and up dedicated ship=to-ship combatant with little to no fighter carrying capacity, in contrast to destroyers (which are hybrid warship/carriers and can lean on either side; case: Orion vs Hecate), and carriers (which have little-to-no direct ship-to-ship options).
In real-world wet navies, dreadnoughts were initially defined as line-of-battle ships with a uniform main battery. That definition kind of degraded as everything started using that. Battlecruisers are defined by being under-armored for their size and armament (something about not being able to resist their own guns), for more speed. At least in WWI, those were the going definitions. By WW2, I guess all those definitions kind of eroded (?). Though from the Nagato vs Renown thing, you can see that the Renown has significantly less displacement despite being bigger, which sounds like its underarmored to me.