It is used in the hacker world to denote someone that is into the retarded type of childish cracking, such as those that ask how to hack hotmail accounts or porn sites, or plays with trojans.
Derived from
the french word 'moutard' which means 'kid' but in a more negative way...sort of like a brat.
But with this spelling it's more of a retarded kind of
childishness.
The proper
way to say this word is with your hand bent at the wrist and slapping your chest when you say the 'moo' part, like a retard would act.