[identity profile] ikkyu2.livejournal.com 2009-12-22 08:00 pm (UTC)(link)
I always assumed it was a Greek capital letter 'chi' - the first letter of the word Christ (or Christos) in Greek.

[identity profile] mskala.livejournal.com 2009-12-27 02:50 am (UTC)(link)
Likewise.