'lifelong' originates from English, specifically the compound of the words 'life' and 'long', where 'life' came from Old English 'līf' meaning 'life, living' and 'long' came from Old English 'lang' meaning 'long (lasting)'.

'lifelong' changed from Middle English compounds such as 'līflonge' / 'liflunge' and eventually became the modern English word 'lifelong'.

Initially, it meant 'lasting for a long time in relation to life', but over time it evolved into its current meaning of 'lasting for the entire life'.