To make sure or certain; to assure or guarantee something with confidence.
From Latin assurare (ad- 'to' + securare 'to make secure'). This archaic or rare variant developed as English absorbed Latin legal and commercial terminology.
This word is basically a fancy, old-fashioned way to say 'assure,' but it's so rare now that most modern dictionaries don't even include it—it's the kind of word a lawyer from the 1600s might have used in a contract.
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