I always thought it just meant someone from the Western areas of Russia and/or current day Belarus.  In fact, doesn't Belarus translate to 'White Russian'?  Lemme look it up...hmmm...under dispute...as are many things with potential political ramifications.  IIRC, it was also a term used by the forces which opposed the communism and presumably other stuff of the Bolsheviks ('reds') until they were finally and comprehensively defeated and ended up in Siberia...if they were lucky.