How did the term for a central European country (long since defunct) become the term for people exhibiting a particular lifestyle?
From the artists living there practising that lifestyle?
Or read Cecilhimself.
Pety nitpick: The country was Boehmen, the people something derived from that, in German – and hence Bohemia/-ians in English. To themselves they were always the Cesky, from what is now the Czech Republic.