It shows up in US Southeastern slang quite a bit. Usually it's a bit stronger than 'going to,' ex. "I'm fixing to give him a piece of my mind." 'Fixings' is also slang for side dishes here.
As a Southern gal myself, I got whiplash from the implication that people sayin "fixin" are trying to sound important.
I promise, among those of us who say that kinda thing, it's seen as a mark of ignorance and bein low class in general. The idea that a hillbilly accent can seem "important" is banana sandwiches to me.
I wouldn't say it's trying to sound important as much as it is trying to sound serious / no bullshit tone: "I'm fixin' to whop yo ass", or as a response to your boss bitching at you to do something: "yeah yeah I'm fixin' to"
It's dialectic -- there's lot's of them in the US, but this one afk belongs to Black American English, and is shortened from "fixin' to." Personally, I think it's cool to see so many variations of English. The language is definitely not static; it is changing all the time!
Usually implies “I’m” fixing to. Often said without much emphasis, as it’s just introducing the important part of the phrase. I think it’s actually a pretty neat way to keep the emphasis where it needs to be.
“Finna get outta here” uses 3/4 of the phrase to convey the important action of “leaving”
vs. “I’m fixing to get out of here” uses 1/2 of the phrase on useless info that “I” am the one doing the leaving and that it hasn’t happened yet but is about to.
Finally gonna is already a slang shortening of "I am finally going to...". Or even better, "I will finally...".
These terms used to bother me too, until I just full-on embraced them. Now I use them both ironically, and unironically, just never at work. They're really good for text messaging because of their brevity. They combine multiple words into a single short word.
Use them ironically with someone close to you. They're quite contagious. They will rapidly grow on you until you find yourself using them unironically too. Just don't use them in situations where professionalism is expected, or clarity is important.
For funsies. Or because it will help you appreciate the words instead of resenting them. That's why I did it. It's just a suggestion though. You know what works for you.