He threw his toys after losing the game—such childish behavior is really frustrating.

childish 是由名词 child（孩子）加上形容词后缀 -ish 构成的形容词。-ish 表示"具有...特征的"，所以 childish 意思是"孩子气的、幼稚的"，通常带有贬义，指成年人表现出不成熟的行为。