(informal)remain silent, especially so as not to reveal a secret
(非正式)（尤指为保密）保持沉默，守口如瓶
he was keeping mum about a possible move to West Ham.
他对可能搬到西哈姆一事不发一言。
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.
在这个意外的集会上别多话