The phrase “keeping mum,” as in “to stay silent,” has its origins in the 14th century, and there are a number of related phrases which also use “mum,” illustrating the many ways in which a language can diverge through daily use. The concept of keeping mum also comes up in some plays on words which utilize the British slang term “mum” for “mother,” which dates to the early 1800s. Propaganda posters in the Second World War, for example, exhorted readers to "Stay Like Dad: Keep Mum" or alternatively, "Be Like Dad: Keep Mum."
“Mum” appears to be a word of imitative origins, referencing the “mmmmmm” sound that people make when their mouths are closed and they try to talk, or when someone tries to talk with a hand clamped over his or her mouth. Since the 14th century, people have been talking about “keeping mum” to stress the idea that they will not spill a secret or talk about an issue. People were also, of course, telling each other to keep mum.
The term was also borrowed by members of the acting community, who started to put on “mummeries,” or silent plays, at around the same time. In a mummery, people would wear masks to conceal their identities, and portray a scene which was often laden with political and social commentary. Actors who participated in such plays were known as mummers, and sometimes high-ranking members of society would even participate, using the disguises as an excuse to get involved in some frivolity.
By 1704, people were saying “mum's the word,” referencing the idea of keeping mum. “Mum” is also linked with “mumps,” a disease which causes a painful swelling of the face which makes it very difficult to talk. “Mummy,” however, comes from a Persian word, mumiya, which references the waxes used to prepare Egyptian mummies for burial, although mummies are certainly also good at keeping mum.
This slang term is used in many English-speaking countries, and occasionally it results in some humorous headlines in Britain, thanks to the fact that “mum” can refer to silence or a mother. For example, the headline “Politician keeps mum on drugs” could be read in two different ways, depending on which “mum” one has in mind. Some newspapers create deliberate double-entendres with this in mind, and at other times entertaining headlines are purely accidental.