to keep quiet about something; my lips are sealed.
mum means mmmmmm (the sound made when putting your lips together)
came from Shakespeare quote:
"Seal up your lips and give no words but mum."
person 1: "Please don't tell anyone about my affair."
A criticism pertaining the management, when criticising middle management or other useless and expensive building hold backs, Gölök Buday, the Black Jester would say "MUMs the word," going back to the 2000election when Maharishi's university of Management started getting noticed, ala John Hageline.
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.