epistleyoudaoicibaDictYouDict[epistle 词源字典]
epistle: [14] Epistle has never really caught on in English as a general term for a ‘letter’ – too highfalutin – but in fact from a semantic point of view its origins are quite simple. It comes ultimately from Greek epistolé, which meant ‘something sent to someone’. This was a derivative of epistéllein, a compound verb formed from the prefix epí- ‘to’ and stellein ‘send’ (as in apostle, literally ‘someone sent out’).

English actually acquired the word for the first time during the Anglo-Saxon period, directly from Latin epistola, and it survived into the 16th century in the reduced form pistle. In the 14th century, however, it was reborrowed, via Old French, as epistle.

=> apostle[epistle etymology, epistle origin, 英语词源]
epistle (n.)youdaoicibaDictYouDict
partly from Old English epistol and in part directly from Old French epistle, epistre (Modern French épitre), from Latin epistola "a letter," from Greek epistole "message, letter, command, commission," whether verbal or in writing, from epistellein "send to, send as a message or letter," from epi "to" (see epi-) + stellein in its secondary sense of "to dispatch, send" from PIE *stel-yo-, suffixed form of root *stel- "to put, stand," with derivatives referring to a standing object or place (see stall (n.1)). Also acquired in Old English directly from Latin as pistol. Specific sense of "letter from an apostle forming part of canonical scripture" is c. 1200.