Of or relating to epistles; having the nature or character of an epistle or letter.
From epistle plus the adjective suffix -ic (as in atomic, tragic, historic). This creates an adjective meaning 'relating to epistles,' competing with epistolary as the adjectival form.
Though epistolic and epistolary both mean the same thing, epistolary completely dominated English—epistolic is so rare that most dictionaries don't even list it, showing how language consolidates around winners.
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