Goddamnit originated from spain where it ment something like, "horsecock" or "damned be thy son". Noone knows how, or why, it has been common in the English language.
From it's originated state, it has become somewhat false in the English language, where many still belive it's just "God Damn It!" without spaces. God himself knows this is wrong.
Today it means something (1.) important, and/or (2.) something worth making a statement over. It can also mean that you are (3.) embarrased over something.
1. We need this in today goddamnit!
2. This will be something that people will talk about, goddamnit!