This is the standard phrase that is used for completing a task, to have sex with a girl, make a smart purchase, etc. In addition it is what the majority of all people in America say instead of that mangled distortion known as Git R Dun.
First widely introduced by Larry The Cable Guy, Git R Dun was apparently an attempt to further destroy the English language, and in an effort to make some Americans a little bit dumber and more irritating. Fortunately, however, most Americans aren't attracted to this horrid slang term, and the majority of us still say "git it done" (or get it done, in proper written form.)
Me: It's GIT IT DONE you moronic, imbred piece of shit!