'naturally-occurring' originates from the combination of 'natural' and 'occur,' where 'natural' comes from Latin 'naturalis' meaning 'by birth' and 'occur' from Latin 'occurrere' meaning 'to happen.'

'naturalis' transformed into the Old French 'naturel,' and 'occurrere' became the Middle English 'occurren,' eventually forming the modern English 'naturally-occurring.'

Initially, it meant 'happening by nature,' and this meaning has largely remained the same in modern usage.