Yet, I'm not sure if that tag is indeed the best solution. Does an "untagged" tag actually make sense? If yes, what would be the best way to describe that tag in the wiki summary?
The untagged tag is used automatically by the engine when a migrated question arrives without tags (because none of its original tags are existing here) or when a question ends up without tags due to automatic tag cleanup (tags which for half a year have only one question are deleted).
When you find such a question, please find some useful tags for it (optimally already existing tags, or tags which you at the same time also can give to other questions) and replace untagged by them.
(I have untagged as my single favorite tag to see such questions highlighted.)
Something based on this information could be used as the tag wiki text, too, feel free to propose a nice text.