In a heathenish manner; in a way characteristic of or resembling heathens; barbarously or irreligiously.
From 'heathenish' (adjective) plus the adverbial suffix '-ly' (from Old English '-lice'). This follows the standard English pattern of forming adverbs from adjectives to describe how an action is performed.
Medieval writers used adverbs like this to intensify their moral judgments—describing someone acting 'heathenishly' was a rhetorical way to delegitimize their entire culture in a single word.
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