E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Mystery or Mysterium.
Said to make up the number 666 referred to in Rev. xvii. 5. This would not be worthy notice, except for the fact that the word mystery was, till the time of the Reformation, inscribed on the Popes mitre.
Almost any phrase or long name can be twisted into this number. (See NUMBER OF THE BEAST.)