Childish means acting immature or silly in a way that seems too young for your age. When a ten-year-old throws a tantrum because they didn't get their way, that's childish behavior, the kind of thing you'd expect from a toddler, not someone old enough to handle disappointment. When someone pouts, whines, or refuses to share just to be difficult, they're being childish.

The word carries a sting of criticism. It suggests someone should know better but chooses to act babyish anyway. If your friend calls you childish after you stick your tongue out during an argument, they mean you're not handling conflict in a mature way.

Childish is different from childlike, which describes the good qualities of childhood: curiosity, wonder, imagination, and innocence. Being childlike means keeping the best parts of being young. Being childish means keeping the worst parts: selfishness, impulsiveness, and emotional outbursts.

Adults can act childish too. When grown-ups have a childish argument over something petty, or when they give each other the silent treatment instead of talking through a problem, they're acting less mature than they should.