Meaning of M.U.

I just learned the meaning of M.U.
I attended an action group session with brothers and sisters of the BCBP last night.
I am not actually in the same age bracket with my brods and sis.
I was amused at how my brods and sis, couples, shared their anxities as they deal with raising their teen-age children.
Their young children come home so early already---early in the morning.
Chlidren don;t respond to their text messages.
It keeps them (parents) awake all night, waiting for son or daughter to come home.
Sometimes, they just fall asleep while waiting for eternity.
In the morning, their favorite question would be "What time did Mike come home last night?"
Another teen age child of another couple is known to have a girlfried.
But in an elevator, a sis saw the teen age son having a lips-to-lips session with another girl in the elevator.
Then for the first time I learned the meaning of M.U.
Another teen age child was teased about having a relationship.
The teen ager said she is not in a relationship.
They are just "M.U."
What is M.U.?
"Mag-Un".