dammit

dam·mit [dam-it]
-interjection Eye Dialect.
damn it (used as a mild expletive).
[Origin: 1905-10]

Thank you, Random House! Sure, it's eye dialect--which doesn't say much for my intellect, as the 'speaker'--but look under 'damnit' on Dictionary.com, and I'm afraid you'll come up empty.

And really, who can question such adept linguists as Blink 182?!

If you want to say 'damnit,' just say 'damn it.' Dammit.