For that matter, was it ever a commonly used term?
Never heard of it until this thread. Googled it and it sounds like a misinterpretation of “gosh darn it”.
Amos McCoy made it popular.
I believe that the last active user of the term was an eccentric millionaire, one Mr. Jed Clampett of Beverly Hills, CA. 
so did Yosemite Sam.
No, but “dadgummit” is. (Seriously, my dad says it.)
Does your dad gum it?
And if so, what is it?
I say consarnit because my brother used to say it as a joke. My (American) better half said she’d never heard of it before she heard me say it.
There’s also “dagnabbit,” which coincidentally is also the name of a major newspaper in Norway.
I know people who say that, and dadgummit, which is admittedly more popular.
Nerts to that!
Actually, I don’t think I ever witnessed an event where Mr. Clampett became so emotionally unhinged as to lose control of his tongue to that extent.
His mother-in-law, Mrs. Daisy May Moses, OTOH, was very prone to such language.
As to the question in the OP, yes, “consarnit” is still widely used in the United States. That said, I understand it to be illegal to say it on Sundays in the town of Possum Junction, OK.
They do allow “Jiminy Cricket!”, but only if you’re over the age of 21 and no womenfolks are within earshot.
No, he’s got all his teeth.
I’ve only used “consarnit” as a joke, but I have seriously used “dagnabbit” a few times. It started as a joke, but occasionally drifts into my regular vocabulary.
Since it seems most like a reversal of “God damn it”, I wonder if it is the uber version. Especially since I’ve seen it spelled dadgommit.
My own encounters with “consarnit” are limited for the most part, maybe even entirely, to B Westerns of the 40’s and 50’s and characters like Gabby Hayes, Al “Fuzzy” St. John, and perhaps other sidekick types.
It might be fun to list all the toned down versions of “goddamnit” that serve the same basic purpose as “consarnit” but my own favorite was used by a friend in my early years: “goddurnit.” I sense he was unclear on what part of “goddamnit” needed the toning down.
For what it may be worth, the heavy usage of “cocksucker” and similar expressions in HBO’s Deadwood was due to the need to have as offensive (to current day ears) a word as possible, where the term most likely used in those olden days (and which seems almost tame now) would have been “goddamnit.” It used to be the epitome of vile language in stage plays, but has long sense been replaced by “fuck” in its many forms. In fact, I have heard “goddamnit” unbleeped on TV where words like “shit” are censored!
It would make for some fun predicting to determine what the fashion for obscenity may be in another 50 years. Anybody want to hazard a guess?
I use consarnit, dagnabit, and dadgumit a lot to replace profanity when I’m around old people, the religious or children, as well as gosh-heckin’ and rassen-frassen.
I’m predicting that in 50 years there won’t be any real profanity, but there will be multiple iterations of politically correct epithet replacements like “the N word.” There won’t be any point in speaking anyway, since information will just get beamed directly to the chip implanted in your hand.
How about Jerry Seinfeld?
Probably in his stand up routine as well, but definately in the episode of, ‘Seinfeld’, called, “The Non-Fat Yogurt”:
"There’s nothing more fun than cursing when you’re a kid. I mean it’s like getting the keys to the car isn’t it?
You’re doing something you’re not supposed to do and that’s all you want.
And there’s nothing less fun than when you’re an adult and having to use those…wholesome curses (with a lot of emphasis) ‘Fudge’ (more emphasis) ‘Sugar’ ‘Con-sarnit’ What the hell is con-sarnit anyway?
I mean you stub your toe and you say con-sarnit you might as well say ‘Yippee’ But if you’ve ever been to a foreign country the first thing you learn are the curse words, right?
You travel halfway around the world to experience some centuries old exotic civilization, the first thing you ask is ‘How do they say doodie here?’".
I believe that these terms are only used in situations where genuine frontier gibberish is called for.
–Chefguy Johnson