Adult people, as opposed to children; used informally to refer to people who are fully developed and mature.
Compound of 'grown' (past participle of 'grow') and 'up,' literally meaning 'people who have grown up.'
Interestingly, 'grownups' is mostly a children's word—once you become one, adults rarely call themselves or each other 'grownups,' preferring 'adults' instead, which shows how language marks the shift from childhood to maturity!
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