In a manner that assumes or shows arrogant presumptuousness; with an air of taking something for granted without justification.
From 'assuming' (present participle of assume, but also an adjective meaning arrogant) plus '-ly.' This word combines the verbal noun form with an adverbial modifier to describe how someone behaves when they act with unwarranted confidence or arrogance.
The word 'assuming' flipped its meaning over time: once neutral (the act of assuming), it became judgmental (assuming you're right about things). 'Assumingly' captures this perfectly—it describes an attitude, not just an action, making it psychologically astute about how confidence can become off-putting.
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