Damn it. But I don't mind either use. If they have to be combined, then dammit. I seem to see "dammit" used more in books when hicks are talking.
Sorry to break this to you but neither are actual words so there is no correct spelling. Option 2 is certainly more logical as that is what you are trying to portray: You are damning something.
According to my dictionary Dutch-English and English-Dutch by Van Dale, a well respected publisher of dictionaries in this country, "dammit" is the correct way to spell it and it is an actual word as well according to that same dictionary. It's an interjection.
Whether or not it is a "real" word, every novel I've read uses "dammit." I've never seen "damnit" anywhere but the internet.
Obviously saying Damnit, would just be modern for saying Damn It, which is of course simply cursing the subject of which you speak....which is how it's often used. What the hell does Dammit mean?
yuh, but would you type "they're, their and there" as just one thing? No you type them as different things because that's the correct way to type them, and as damnit is the correct way to type it, it wins (even though they sound the same when you are talking)