B1 adjective /ˈtʃaɪl.dɪʃ/
Childish
子供っぽい; 子供のような振る舞いだが大人には不適切
Meaning
immature and silly; behaving in a way that is typical of a child but inappropriate for an adult
Example
His childish behavior during the meeting was quite embarrassing.
会議中の彼の子供っぽい行動はかなり恥ずかしいものでした。