'grown-up' originates from the verb 'grow' combined with the past participle 'up', where 'grow' meant 'to increase in size or maturity'.

'grown-up' changed from the Old English word 'growan' meaning 'to grow' and eventually became the modern English word 'grown-up'.

Initially, it meant 'someone who has grown', but over time it evolved into its current meaning of 'an adult'.