This is a commonly used English expression which does not relate per se to mothers. This article will explore when it should be used, from where it derives and will give some examples in context.
A person will say “mum’s the word” in order to tell someone to keep quiet. An example of this would be if they have told someone a secret and they do not want it to be repeated.
The expression came into use in the 16th century in Shakespeare’s Henry VI, Part 2 which reads “seal up your lips and give no words but mum.”
Later, in 1720, the current wording of the expression was adopted and it has been frequently used ever since. This was in a piece entitled A Walk Around London and Westminster – The Works of Mr. Thomas Brown which reads “but Mum’s the Word – for who would speak their Mind among Tarrs and Commissioners.
An example of the expression in use can be found in the title of an article in The Guardian newspaper. The title reads “Mother’s Day: Mum’s the word as we celebrate literary mothers.”