` Use 'dammit' or '**** it'. Everybody should bookmark the OneLook site, which searches a large number of free online dictionaries. Click here to read about 'dammit' and then change the headword to 'damnit' for some pretty conclusive evidence, Rover
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My opinion is that the correct spelling is "damnit". I know this spelling from years of reading comic books. If I were writing something more formal, then I'd just avoid using the word/phrase altogether.