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1. (noun) Maturity. The state of being a mature, stable adult. "Mature" is to "grownup" as "maturity" is to "grownupitty".

2. (adjective) Having an aloof, snobbish, or condescending attitude toward "young whippersnappers" after having reached an advanced age, like thirty or more.
Buying a home is a sign that you have reached grownuppity.
by Pod Tailin' January 26, 2011