I'm thinking they mean in the sense that it's not in mainstream use. Think of "underground" music which is played in clubs that are open and welcoming to the pu

I'm thinking they mean in the sense that it's not in mainstream use. Think of "underground" music which is played in clubs that are open and welcoming to the public. One definition when I looked up the word was "a group or movement seeking to explore alternative forms of lifestyle or artistic expression" which I think also applies here.