This informal expression means keep it secret or do not say anything about it. People use it to ask someone to keep quiet about particular information, or to promise that they will not share it.
The key to its history is the word mum. It has meant “silent” for centuries and is linked to the “mmm” sound made with closed lips. That gives the expression a simple connection: closed lips suggest saying nothing. Here, mum has nothing to do with the British word for “mother”.
The older meaning also survives in keep mum, which means to remain silent, especially about something private. The familiar expression turns that idea into a short instruction or promise: silence is what is required.
Examples:
- “We’re planning a farewell party for Ben, so mum’s the word until Friday.” This asks someone not to reveal the surprise.
- “I won’t tell anyone about your plans—mum’s the word.” This promises to keep the information private.
- “She kept mum about the results until the official announcement.” This uses the related expression to describe someone’s silence.
The phrase usually has a friendly, conversational tone. It fits everyday situations such as surprise parties, gifts, or private plans. In a formal message about sensitive information, Please keep this confidential is generally a more suitable choice.