A person who is fanatically obsessed with a geeky hobby to the point of being socially clueless - poor hygeine, rude behavior, moronic sense of humor, arrogance despite obvious foolishness.
Common subcategories include comic book zugs, gaming zugs, Star Trek zugs (trekkers, the people who are so into it they can't let themselves be called trekkies), rennaizugs (idiots with sticks wearing sheets, socially clueless people who live at the Rennaissance festivals and fairs), Rocky Horror zugs (way too late, way too lame), Vampire zugs (fang tards), anime zugs, etc.
This is not a term for the general fans of these hobbies, but for a specifically severe type of social retard.
To my knowledge this was originally a comic book or game shop insider term as early as the 80s, but it has not been shared with the fan base, instead being supplanted by the less derogatory and more gender-specific fanboy.
We'd better get the subscribers comics out of the shipment before the hordes of slobbering zugs come through the door.
That guy seems to think he's cool, but he's just a zug.
You should see how many toys he has in his living room.

A low-level Awk(?) from Geac. (1995-01-23)

The Zug massacre took place on 27 September 2001 in the city of Zug (Canton of Zug, Switzerland) in the canton's parliament. Friedrich Leibacher shot dead 14 people before killing himself.