Button 是纽扣，lip 是嘴唇，Button one's lip 字面意思是“嘴唇装上纽扣”，真实含义是保持缄默，闭口不谈，守口如瓶。

【例句】
He didn't want to get into a fight, so he decided to button his lip.
他不愿意卷入一场争执，于是决定保持缄默。

Sometimes he would sit silent and abstracted, buttoning his lip.
有些时候他神思不定地坐在那里，一言不发，任何人都不理。