In a manner characteristic of French people, style, or culture; done in the French way.
From 'French' with the adverb-forming suffix -ly. Creates an adverbial form to describe actions or behaviors performed in a French manner.
Adding '-ly' to 'French' creates an adverb, but it's interesting that we rarely actually use this word—it shows how some grammatically correct words just don't fit naturally into how we really talk!
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