Believed by non-rednecks to be an interjection. Believed to have originated in 1934 in a small town in Alabama called Mahsister(the name of the town comes from the founder who named it after the one who he liked having intercourse with most). In Mahsister, Alabama, in the summer of '34, a cat had climbed up a tree at one of the local drunken shindigs. The owner of the cat, standing beneath the tree, pointed at the cat while exclaiming "get her down!" The local hicks interpreted this as a celebratory interjection such as right on or yee haw. The locals began shouting the conflation of the phrase; "git'rdone" to rejoice and take part in a shared excitement.
Redneck: Hey Cletus an' Buford...Git'rdone!
Cletus and Buford: GITRDONE!!