"MU" (Pronounced Moo) is short for Mentally Undressing.
It is when a person is flirting with a member of the opposite sex whilst staring at their physical form. A witness or a friend of the person flirting must then say aloud the word "MU".
Two friends witnessing a man flirting with a woman.
Friend 1: "That guy is totally MU-ing that woman"
Friend 2: "I know...MU!"
A friend witnessing a man flirting with a woman.
Friend (aloud): "MU!!!"