Numerology assigns a value to each letter of the alphabet: the A is 1, the B a 2, and so on up to the ninth letter, where it starts over with 1 for J, 2 for K, etc. Following this method, we find three letters with a value of 6: F/O/X. The number 666 translates to FOX and only to FOX. No other English word deciphers to 666.
words courtesy of H. Decoz, World Numerology