From what I've been told, it's an old British name.  Spelled "St John" but pronounced "Sinjin".   It's not common in the UK, but they've heard of it.
  After I heard it on the news, the first time in my life, I kept hearing it again!
  I saw it in a Bond film, 007 was undercover and when his name was read aloud they mis-prounounced it Saint John.  He then corrected them in his sultry way, Smithe, 'Sinjin' Smithe. 
  It was the same name the new priest had such a hard time pronouncing in Four Weddings and a Funeral.