Meaning

adjective written in the form of or carried on by letters or correspondence

origin1627, from Latin epistolārius, from epistola (“letter”) (English epistle) + -ārius, from Ancient Greek ἐπιστολή (epistolḗ) from ἐπιστέλλω (epistéllō, “to send a message”) from ἐπί (epí, “upon”) + στέλλω (stéllō, “to prepare, send”). For the noun, compare Middle English pistelarie and Old English pistelari, both from Medieval Latin epistolārium, possibly via an unattested *epistelari, *epistolarie.

Etymology from Wiktionary, CC BY-SA 3.0