(informal)remain silent, especially so as not to reveal a secret
(非正式)（尤指為保密）保持沉默，守口如瓶
he was keeping mum about a possible move to West Ham.
他對可能搬到西哈姆一事不發(fā)一言。
Say nothing of the secret you know
保密：保守你知道的秘密
Mum's the word on the surprise party.
在這個意外的集會上別多話