The quality or characteristic of being arrogant and presumptuously taking things for granted without proper justification.
From 'assumptious' plus '-ness.' This creates an abstract noun naming the quality itself. Since 'assumptious' is already quite rare, this word is even more uncommon, appearing almost exclusively in formal or archaic texts.
This might be the most rarely used word in the assume-family—it's so obscure that searching for it returns primarily scholarly articles rather than actual usage. It's a perfect example of English's infinite generative capacity: you can technically create almost any word you want through suffixation, even if nobody actually uses it!
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