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.
I am fixing to take my shower. Anyone need in the bathroom before i do?
or
Dude, watch this, Rocky is fixing to kick Appollo Creed's ass.
or
Im fixing to go out , so don't wait up for me.
Nickname given to bales of marijuana thrown overboard or out of airplanes. Origin is reportedly from members of the US Coast Guard in the area of Southern Florida.
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.
I had a trucker's tan after driving for 10 straight hours on a sunny day.
British slang for a scientist or technical expert, esp . when the speaker is not certain of the expert’s specific field of study. Not necessarily derogatory, but typically used by persons without a technical background. In the US, the equivalent would be “egghead”, “squint”, or “geek.”
Origin of the word is unknown. Started sometime in World War 2. Given the typical evolution of British slang, could easily have started because what these experts did was confusing or “baffling” to the common person.