GIT is the Southern pronunciation of GET (out of here!) and is what you say to children when they're all in the kitchen, for example, and you want to cook. "You all git from here - I need to finish dinner!" -or- to a dog that you want to go outside: "You better git before you master sees you in here tracking mud!" It's basically short for "git out!" Additionally, in some parts of the South (especially amongst the colored population) it was customary to put the syllable a- in front of the progressive: "The boss is a-comin'" (also to reduce -ing to -in', which a lot of speakers still do in speech (including me)). It doesn't change the meaning, it just makes it more ... Southern :) So this expression means "I'm a-gettin' (out of here)" when the speaker perceived trouble. (I'm not going to stick around and see what happens).