Mum

—see Mom
An interjection directing silence. Mum for that; I shall be silent as to that. As mute as Mumchance, who was hanged for saying nothing; a friendly reproach to any one who seems low-spirited and silent.
“to keep mum ,” to hold one's peace. Hence “mum's  the word,”—a phrase which implies to all hearers that the matter to which it refers must remain secret.