In an archaic or poetic manner, with hardness or endurance; in a lasting or persistent way.
From dure or durable with the adverbial suffix -ingly, following Old English patterns for creating adverbs. This is an archaic construction rarely used in modern English.
Archaic adverbs like 'duringly' are time-travel documents—they show us how English speakers once described persistence and strength before modern words like 'enduringly' became standard.
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