She said staying up late was for the project, but it was just a rationalisation to cover her binge-watching habit.

rationalisation 是名词，来自动词 rationalise（使合理化）。rationalise 由词根 rational（理性的、合理的）加上动词后缀 -ise（使...化）构成，意思是"使某事看起来合理"。 rationalisation 指"合理化过程"或"合理化解释"，在这里表示"用看似合理的理由来解释自己的行为"，实际上是为了掩盖真实动机。