Rootin'-tootin simply means highly enthusiastic or boisterous. We don't use it in any English unless we're writing a book about the Old West {the US}. It's merely an old saying that was used for fun. You might hear it in certain situations. Maybe someone is throwing a party and they use it. { Or you'll hear it in the Simpsons} It's good to know but not an essential part of your vocabulary in any way. By the way, it's rootin'-tootin, you always drop the g in each word.

It means "energetic and enthusiastic." It is informal. It is a regional expression, which I think of as being associated with the American West, and possibly from a long time ago. I don't think I've ever heard it used in real life outside of the movies. I don't know if people still use it today in the Western states. I live in the East and I'm a city boy, so I would not normally use it, except as a kind of joke. For example, if my eight-year old grandson decided to dress in a cowboy costume for Hallowe'en, I might say to him, "Well, aren't you a rootin'-tootin' six-shootin' cowpuncher! Howdy, podner! I'll bet you're off to lasso yourself some candy!" Stephen King is using it as a kind of joke, here. The word is an adjective. It's not a noun. His making a noun out of it is a joke. It is as if someone said "The colorized version of 'It's A Wonderful Life'--now with 40% more wonderful in it!" The phrase "with X% more Y in it" is a frequent pattern in advertising claims. Since the word doesn't exist as a noun, you can't really find a "perfect synonym" for it. If you insist, "action" or "excitement" or "energy" might be a synonym for the way King is using it. oxforddictionaries has an entry for it and (of course) define it as an adjective. They define it as "Brashly or boisterously enthusiastic." https://en.oxforddictionaries.com/definition/rootin'-tootin'