In a manner that is stretched out or prolonged; in a way that covers a large area or long period of time.
From 'extended' (past participle of extend from Latin extendere) plus the adverbial suffix '-ly', meaning 'in the manner of.' The root extends back to Latin ex- (out) and tendere (to stretch).
This word shows how English can turn almost any adjective into an adverb with '-ly'—it's like a universal adapter for language! Even less common words like 'extended' get this treatment, which is why 'extendedly' exists even though most people would say 'in an extended way' instead.
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