Fixing
This "fixing" has nothing to do with repairing something. In some Southern dialects you can say you're fixing to do something if you're on the verge of doing it, like in the sentence "They were fixin' to leave without me."
The construction comes from an older version of the word "fix" meaning to prepare. Similarly, "fixings" are all the trimmings and side dishes that accompany a meal.
Barbecue
As anyone from the South can tell you, barbecue both a type of grill and the food you make with it — usually pulled pork coated in spices and sauce and cooked over a flame.
There are several regional variations of barbecue, with North Carolina, South Carolina, Memphis, Kansas City, and Texas each claiming their own distinct method of preparation.
Whatever you do, don't use "barbecue" to refer to a gathering where barbecue is consumed — that's a "cookout" in the South.
Buggy
In most parts of the country, a buggy is a little carriage pulled by a horse.
In the Deep South, you'll only find buggies in the grocery store — it's another word for "shopping cart."
Mash
In some parts of the South, they don't "press" the buttons in an elevator or on their remote control — they "mash" them.
The word is so distinctively Southern that one Dixie blogger called it the "ultimate secret handshake of the Southern lexicon."
Yankee
What Yankee means is completely relative to where in the world you are.
In the UK, a Yankee is an American. In the northern US, a Yankee is a New Englander.
And in the South, a Yankee is a northerner. Suffice it to say it is not a loving term.
Sweeper
This is one that the Midwest and the South can claim together — both regions have dialects where people say "sweeper" to mean a vacuum cleaner.
Foot
Lifelong Southerners won't find it strange to hear "foot" used as an interjection when something goes wrong. As in, "Oh, foot! Auburn lost again!"