Late Middle English; earliest use found in Vegetius' De Re Militari. As adjective from classical Latin mīliārius, mīlliārius comprising a thousand (frequently designating military units), a thousand paces long from mīlle thousand + -ārius. As noun in sense A. 1 from classical Latin mīliārium, mīlliārium milestone, use as noun of neuter singular of mīliārius, mīlliārius, with reference to the fact that a mile comprised a thousand paces. As noun in sense A. 2 from post-classical Latin miliarius, milliarius, use as noun of masculine singular of classical Latin mīliārius, mīlliārius, after Byzantine Greek χιλιαστής.