Is it idiomatic to say "would be highly probably"?

Question

In my other post ("The question is that" vs. "The question is") I said

I guess the 3rd way ... would be probably grammatical and idiomatic.

And I feel I could convey more confidence. Then this expression came out

I guess the 3rd way ... would be highly probably grammatical and idiomatic.

I am not sure whether it is grammatical and idiomatic. so, I search on ELL and Google Ngram

So, is it grammatical and idiomatic to say "would be highly probably"?

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Before @Astralbee @KateBunting and @WeatherVane mentioned the other possibilities, I didn't consider highly probable, highly likely, almost certainly. Here is the update.

Answer 1

The problem with your use of an ngram here is that you have compared a two-word phrase with a single word. Obviously the single word will have more results because it will be found in all possible contexts. In fact, any uses of "highly probably" found will also be included in the "probably" results. Low-usage alone of a word or phrase does not make it wrong - you would need to look at some examples context.

Most of the examples of "highly probably" found in Google books seem like mistakes to me, and should have said "highly probable". However, I am struggling to discount it as incorrect.

For example:

It is highly probable that this is idiomatic.

I believe this could also be written as:

This is highly probably idiomatic.

What might be worth noting is this ngram in which I compared the phrase "highly probable" with "highly likely". It is interesting to note that use of "highly probably" has fallen over time, while "highly likely" has increased. Perhaps use of an adverb to exaggerate probability is not so common in either case?