In his book Fire Weather John Vaillant, who is great at these kind of factlets, gives us this:
Words possess spell-casting, shock-inducing power, even in this jaded age, and the English language has accounted for this: something that is “infandous” is a thing too horrible to be named or uttered.
I’ll admit that when I first read this I thought perhaps Vaillant had meant “infamous” and that there had been a typo. Even spellcheck tells me it’s a mistake. But no, infandous is a word. Dictionaries define it as something “too horrible to mention,” or “extremely odious.” It derives from the Latin infandus, “not to be spoken of.” Apparently Increase Mather spoke of things being nefandous, which meant the same thing but wasn’t as popular.
All of that was, however, long ago. The Oxford English Dictionary entry for infandous tells us “This word is now obsolete. It is last recorded around the early 1700s.” I did some digging and found where it had been used more recently than that, but I don’t think you’re likely to see it around much today.
Alex Good 
What a great word, should still be in use! I might have to unobsolete it!
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😁
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And what does Mr. Vaillant think is infandous?
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It’s a book about the Fort McMurray wildfire in 2016 and “for a mayor or a fire chief, a fire running rampant through the city they are charged with protecting is infandous.” Which actually isn’t how I would use it, but I can see it.
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Yeah, that’s less unspeakable than unthinkable.
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I think a part of the problem with using it today is that even if you have a dictionary definition, because it’s obsolete and has been for so long we don’t have any generally accepted usage for it in our heads. If you just take it as meaning “something really bad” you could see it as fitting, but “something so bad you can’t speak of it” doesn’t work for public officials in a state of emergency addressing the media. You have to have a catalogue of examples of its use in your head though to decide if its appropriate, and we don’t.
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I like the word, but my goodness, it is a bit unwieldy. Unspeakable seems to flow better…
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I feel the same way, but think part of our problem may be that we’re just not used to it.
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That definitely doesn’t help. But having that “d” in the middle makes it stick as you’re saying it.
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Yep.
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