The present-day Czech Republic incorporates what were once known as Bohemia, Moravia, and Czech Silesia. “Bohemian” may be used to refer to any citizen of the Czech Republic today, or of the language they spoke before “Czech” became more prevalent in English.
The lower-case noun "bohemian" is used casually today to refer to a person, often an artist or a writer, who lives and behaves in a free manner, unconstrained by conventional rules and mores. It emerged in 19th-century France, when writers and artists started to gather in lower-class, gypsy neighborhoods, and "bohémien" was a common term for Romani people who had migrated west to France via Bohemia. Puccini's wildly popular 1896 opera "La bohème" may have had a hand in the evolution of the term.