She tried to rationalise forgetting her friend's birthday by saying she was too busy with work.

rationalise 是动词 rationalize 的英式拼写形式，由词根 ratio（理性）和后缀 -al（形容词后缀）以及 -ize（动词后缀）组成，意思是"使合理化"或"为...找合理借口"。这个词指试图用看似合理的理由来解释或证明某种行为或决定。