Mother
A long time ago, in a galaxy far, far away, the term "mother" meant something. Or so the story goes. Few people, today, are sure what that meaning may have been.
There is a rumor that an endangered species of humans, called Christians, have a concept of "mother" that is connected in some mysterious way with another concept they call "Church." When the curator of the World Museum of Natural History was queried as to what this might be, however, its response was "I dunno. You got me hangin'."