In a duncish manner; in a way that shows foolishness or ignorance.
From 'duncish' with the adverbial suffix '-ly,' which converts adjectives into adverbs describing how an action is performed. This is one of the most common suffixes in English.
English makes it easy to describe how someone does something foolish—you can go from 'dunce' to 'duncish' to 'duncishly,' each step adding a grammatical layer that lets you be more and more specific about stupidity.
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