"puerile" in the adjective sense
1. puerile
of or characteristic of a child
"puerile breathing"
2. adolescent, jejune, juvenile, puerile
displaying or suggesting a lack of maturity
"adolescent insecurity"
"jejune responses to our problems"
"their behavior was juvenile"
"puerile jokes"
Source: WordNet® (An amazing lexical database of English)
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