'average-length' originates from the combination of 'average' and 'length', where 'average' meant 'typical or common' and 'length' referred to 'the measurement of something from end to end'.

'average' and 'length' have been used in English since the Middle Ages, with 'average' deriving from the Old French 'avarie' and 'length' from the Old English 'lengðu'.

Initially, 'average' meant 'customary' and 'length' referred to 'distance', and this meaning has largely remained the same in modern usage.