callistosh65 wants to know, "How do you spell dammit/goddamnit (including for British fandoms)?" Hooray for questions about cussing!
(With examples from The Rocky Horror Picture Show and Star Trek.)
My dictionary of choice (Merriam-Webster's Collegiate Dictionary, 11th Edition, natch!) does not have an entry that covers this. Stymied, I had to turn to a greater authority:
Brad: The river was deep but
I swam it. (Janet)
The future is ours so
let's plan it. (Janet)
So please don't tell
me to can it. (Janet)
I've one thing to say
and that's dammit, Janet,
I love you.
The Rocky Horror Picture Show soundtrack says "dammit," and that's good enough for me! But if you want a more reputable source, Random House and American Heritage dictionaries (which reflect common usage more than the M-W) both have an entry, and they spell it "dammit."
There does not seem to be any difference between American and British usage for this one, as the Oxford English Dictionary spells it the same way.
My M-W does not include an entry for "dammit" because this is a pronunciation spelling (or eye dialect) of "damn it." That is, a nonstandard spelling, often contractive, meant to indicate how the word is said aloud. Other examples include "gonna" and "y'know."
This phonetic element helps to explain why the word is spelled "dammit." If you are trying to render the way in which a word is pronounced, why include the silent "n" that is merely a vestige of its original Latin? I confess, when I see "damnit" in a fic, I mentally pronounce the "n" and end up with the regrettable "dam-nit."
Pronunciation spelling can be appropriate in writing dialogue. However, you must weigh the value of using pronunciation spelling versus standard in terms of characterization and pace. Pronunciation spelling tends to convey a rapid, informal speech style that is not always appropriate. Consider the apocryphal words of Dr. McCoy.
Damn it, Jim, I'm a doctor, not a [insert any- and everything here].
In playing the curmudgeonly Bones, DeForest Kelley never strayed too far from his dialectal roots in the Southern U.S. Even in moments of high emotion, his endearing grumble is better represented as two words. Listening carefully to your characters will help you to best convey their voice to your readers.
And this brings us to "goddamnit." The only dictionary I found to list this entry was Random House Webster's. The same Random House that lists "dammit" with two Ms spells "goddamnit" with an N, only listing "goddammit" as a variant spelling. Why? There aren't always concrete answers when it comes to usage, but I would venture to say it's because with three words crammed into one, maintaining the correct spelling of each word makes the whole easier to read.
That said, I believe there is a reason only one of the dictionaries I consulted has an entry for "goddamnit." It is not an intuitive pronunciation spelling: It's difficult to say rapidly enough to justify the form. Unlike "dammit," with its plosive burst and second-syllable drop, "goddamnit" takes time. Either the D or the M will almost invariably slow you down. Here, even more strongly, I would recommend that you really listen to the character before venturing into a pronunciation spelling of the phrase. Most often, you're probably better off writing it as "god damn it."