Английский Этимологический словарь - average (n.)

- 1540, from O.Fr. avarie, from It. avaria, from Ar. 'arwariya "damaged merchandise," from awar "damage." Original sense was "financial loss incurred through damage to goods in transit;" meaning shifted to "equal sharing of such loss by the interested parties." Transfered sense of "statement of a medial estimate" is first recorded 1735. The adjective is first recorded 1770; the verb is from 1769. The mathematical extension is from 1755.