Why do we say "Mum's the word!"?

Meaning:

Keep it secret

Background:

"Mum's the word" has been a popular expression since at least the 17th century; for example:

"Seal up your lips, and give no words but mum." - Shakespeare, Henry VI, Part II, I, ii.

Mum, with the meaning of silent, is thought to originate in Middle English - the name linguists give to the diverse forms of the English language spoken in England between the Norman invasion in 1066 and about 1470.

The OED defines 'mum' as an inarticulate sound made with closed lips.

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