The quality or state of being astonishing; the degree to which something is surprising or amazing.
From 'astonishing' (present participle of 'astonish') + '-ness' (forming abstract nouns). A compound formation that's grammatically sound but rarely used in favor of 'astonishment.'
When you add '-ness' to an adjective, you get an abstract noun—'astonishingness' proves English will let you create almost any noun you can imagine, even if nobody actually uses it.
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