say it: GROH-nuhp
grownup is a noun and adjective meaning "a fully developed person from maturity onward". It carries 2 recorded senses.
a fully developed person from maturity onward
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for grownup come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.