It’s not recognized by Firefox’s spellcheck. It’s not listed in any dictionary.com dictionary. But for the life of me, I can’t see why it shouldn’t be. Can anyone enlighten?
Doesn’t seem unusual to me, although it was flagged by Firefox.
Yes, it’s totally proper.
dictionary.com doesn’t list many regular contractions (e.g., you won’t find “would’ve” or “could’ve” either. And the latter, Firefox flags as well. Firefox’s spellcheck dictionary isn’t in any sense exhaustive)
Much better than “should of” in any case 
Which spell checkers of course have no problem with.
I would call it “proper colloquial,” as opposed to “proper highfalutin.”
I wouldn’t’ve gotten my knickers in a twist about it.
And to think that I constantly use contractions such as “I’d’ve” without blinking an eye.
Wiktionary includes it in their list of contractions. I wouldn’t consider the page authoritative, but it is interesting none the less; particularly for the entries of double and triple contractions.
Yup. You mustn’t be shy about using it. Oughtn’t you?
Huh, I wouldn’t’ve thought so.
Huh. It shoulda.
What is a “proper contraction” anyways?
It’s as opposed to a Braxton-Hicks.
Many contractions are flagged by Firefox on my machine.
Do you use it in writing, or only in speech?
It never occurs to me to use “I’d’ve” in writing. I wrote “I’d have”. I do speak it a lot, but I consider it to be a slurring, not a contraction.
Thank you. God, I have so few grammatical pet peeves, but this one is by far the biggest.
I weep for humanity.
I amn’t the only one who uses it then?
Whaddya talkin’ about?
That’s a whole nother issue. Besides, ‘‘it’s’’ is a contraction and not a possessive. Didya know that? Many don’t.
Damn, I could’ve had a V-8. Where’s the Beef?
My kids use “amn’t.” They came up with it themselves. Which makes me wonder why this isn’t proper. Why is “aren’t” proper, but “amn’t” not? It fits the pattern: isn’t, aren’t, amn’t.