His puerile behavior at the formal event was truly disappointing.

puerile 来自拉丁语 puer（男孩），加上形容词后缀 -ile 构成，字面意思是"像男孩一样的"，引申为"幼稚的、孩子气的"，通常带有贬义，指行为举止不符合年龄应有的成熟度。