Relating to, characteristic of, or resembling heathens or heathenism; pagan or non-Christian.
From 'heathen' plus the adjectival suffix '-ly' (Old English '-lic'), which originally meant 'like' or 'of the nature of.' This is a somewhat archaic form compared to 'heathenish.'
The '-ly' suffix is tricky—it usually makes adverbs from adjectives ('quick' → 'quickly'), but here it makes an adjective ('heathen' → 'heathenly'), showing how flexible and inconsistent historical English formation could be.
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