An adult person, or (as an adjective) referring to things suitable for adults.
Compound word from 'grown' (past participle of grow) and 'up' (preposition). It's a relatively modern word in English, becoming common in the 1800s as a noun meaning adult.
Kids call adults 'grownups,' but the word actually contains biology—it reminds us that adults are literally people whose bodies have finished growing. Some scientists study why people keep acting like kids emotionally even after they're 'grownups' physically.
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