1. the quality or state of being childish; behavior or thinking typical of a child rather than an adult
Usage: often used to criticize immature behavior in older children or adults

Examples

• His constant complaining and refusal to compromise showed a lack of maturity and childishness.
• The teacher was frustrated by the childishness of the teenagers who kept making silly jokes during the lesson.
• She outgrew the childishness of her early years and became more responsible.
• His childishness made it difficult for him to maintain serious relationships.
• The movie's humor relies on childishness and slapstick comedy.
• Despite being an adult, he displayed childishness whenever he didn't get his way.