The Nedmother (or Nerdfather) plays a role in a childs life similar to that of a Godparent, the significant difference being that the Nerdmother is responsible for the teaching of all things nerd related.
Primary roles include the taking of said child to science museums, conventions, comic con etc, and making sure the child grows up on a healthy diet of science fiction shows.
The Nerdmother should make sure that birthdays include at least one nerdy present, such as a giant pair of Hulk hands or a replica light saber.
Mike: "Hey, Georgia, wanna be my kids Nerdmother?"
Georgia: "Are you frelling serious? You're fracking right i do!"