In a manner that makes a confident assertion or claim without doubt or qualification.
From assertory (characterized by assertion) + -ily (adverbial suffix forming manner). Both derive from Latin asserere meaning to claim or state firmly.
This is the rarer cousin of assertorically—both exist in English but assertorily is more old-fashioned, yet it's delightfully precise for describing someone who's absolutely sure of themselves, even when they shouldn't be!
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