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infancy
childish; infantile; immature; primitive
彼の言動はまるで幼稚だと思われた。
His words and actions were considered utterly childish.
大人の割に彼の考え方はまだ幼稚だと思う。
I think his way of thinking is still immature.
子供らしい幼稚な遊びが懐かしい。
I feel nostalgic for the childlike, innocent games of youth.