British spelling: to write epistles or letters; to compose or communicate by means of epistles.
From epistle plus the verb suffix -ise (British) or -ize (American), which converts nouns into verbs. Similar to criticise/criticize or organise/organize.
British and American English diverged on the -ise/-ize split during the 18th century for philosophical reasons—and obscure words like epistolise preserved that difference as a living fossil in spelling.
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