Most heathen; showing the greatest or most extreme characteristics of heathens or paganism (archaic usage).
From 'heathen' + superlative suffix '-iest', which indicates the highest degree of a quality. In older English, '-iest' was freely used with 'heathen,' though this form is now archaic.
The superlative '-iest' only works on short adjectives that fit the pattern—you can say 'warmest' but not 'beautifulest,' which is why we naturally developed 'most' for longer words. It's like English developed a two-tier system of comparison without anyone planning it!
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