Is "I'd've" proper use of the English language?

While reading a book, I came across the word I'd've, as in:

I'd've argued against it.

While it was obvious what it meant, it left me puzzled. Is I'd've a proper word?

After reading your post, I realised that I say "I'd've" quite a lot in my actual speech. But I have never ever written it down, nor have I seen it written down (or, more accurately, I don't recall having ever seen it written down.)

It's not the kind of thing that I'd feel comfortable putting into a business email, definitely not an essay (unless that was my topic, oh, and I think that will be the topic of my next essay now.)

But it is the sort of thing which would fit nicely in the dialogue inside a novel. And you never know, it could one day be perfectly cromulent to write that, and would perhaps embiggen the written English language.

There are 49 incidences of I’d’ve in the Corpus of Contemporary American English (search for I 'd 've). All but one occur in dialogue in fiction. The other one was in a transcript of Oprah.

It doesn’t appear that I’d’ve has any substantial contemporary usage in nonfiction writing at any level of formality. Of course, people say I’d’ve all the time, but if they were to write it down, they’d probably write I’d have.