so..who is "MUD"?

lol, I guess I missed something a while back. Sometimes when I'm reading posts, other people will comment something like, "this is ridiculous, this must be MUD...  this has to be MUD...."

so I'm asking...who or what is MUD??   And for the record, I feel kind of bashful writing this, because my brain keeps telling me "DUH! mud is water and dirt!" lol      

so ladies, what am I missing??