I’ve never figured this out. I get funny looks every time this perfectly sensible phrase, used by millions of people, comes out of my mouth in the presence of non-Southerners. “I’m fixin’ to go to the store”, “I’m fixin’ to cook supper”, “I’m fixin’ to watch the Saints game”, etc. Perfectly understandable.
Had a Yankee boss once who loved to make fun of me for saying that. So I “fixed him” by substituting “I am now preparing to….” in a robotic manner, and he got sick of that pretty quickly. So what do they say?? Who knows – gotta run, I’m fixin’ to go get some more coffee.