To do, finish, or accomplish something—especially that which is difficult, unpleasant, or undesirable—as promptly or quickly as possible so as to no longer have to think about or deal with it. I absolutely hate getting blood drawn, so let's just get in there and get it over with! I'm looking forward to getting my final exams over with so I can relax and enjoy my summer break.

An exhortation to do something quickly (typically because it is unpleasant). I know you have to draw blood, so go ahead, get it over with! It looks like you have some bad news for me, so just tell me what it is—get it over with.