His bad behaviour was often excused, being blamed on his boyish vitality.

vitality 是名词，来自形容词 vital（充满活力的、至关重要的），加上名词后缀 -ity 构成，表示"生命力、活力"。 boyish vitality 指"像男孩一样的旺盛精力"，常用来形容年轻人充满活力的状态。