to add in a small specific detail in order to make your lie seem more convincing
Jim: "So he broke into her apartment, jumped her, tied her up, and kidnapped her. Oh, he also stole a pet snake."
Tim: "Hm, this story must be completely credible because a random, unnecessary detail was added in. Wait a second... he must've been faking the snake!"
A filing cabinet is formal term for a person that exhibits little to no intelligence, provided that they have a functioning brain. Many people mistakenly use terms like "stupid", "retarded" or "mentally handicapped" in place of such a word, but those words are reserved for people without a fully functioning brain.
It means you are about to do something, or more accurately you are doing something now that will lead to you doing the thing that you are fixin' to do. Fixing = preparation
Others here have the pronunciation wrong, when spoken it sounds like one word: "fixina" which led to it's child word finna In the South we like to run our words together, it's too hot to pronounce every letter.
Texan: "I'm fixing to go unload my pickup cuz it's full of junk and I've gotta go get cokes for the party."
Translation: I am in the process of preparing to go begin unloading the miscellaneous items from the back of my truck because I must go purchase beverages for the party.