Hackers tend to be sensitive to logical inadequacies in language, and many have adopted this suggestion with enthusiasm. The word "mu" is actually from Chinese, meaning "nothing"; it is used in mainstream Japanese in that sense, but native speakers do not recognise the Discordian question-denying use.
Thanks to Thrakem for the source of this definition.
Joshu retorted, "Mu!"
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".
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!!!"