In a way that shows no belief in or respect for God; acting without religious faith or moral principle.
From 'godless' (adjective, from Old English 'god' + '-less' suffix meaning 'without') + '-ly' adverbial suffix. The compound evolved to describe both atheistic behavior and immoral conduct.
In the 1600s-1700s, calling someone 'godless' was one of the worst insults possible—worse than almost any other criticism—which shows how deeply religious authority was woven into social respectability.
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