E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The four letters, meaning the four which compose the name of Deity. The ancient Jews never pronounced the word Jehovah composed of the four sacred letters JHVH. The word means I am, or I exist (Exod. iii. 14); but Rabbi Bechai says the letters include the three timespast, present, and future. Pythagoras called Deity a Tetrad or Tetractys, meaning the four sacred letters.
The words in different languages:
East Indian, ZEUL and EAI.
Egyptian, ZEUT, AUMN, AMON.
Hebrew, JHVH, ADON.
Persian, SORU, SYRA.
Swedish, OODD, GOTH.
Such was the sacred Tetragrammaton.
Things worthy silence must not be revealed.
Dryden: Britannia Rediviva.
[We have the Egyptian Θυ, like that Greek Θεo.]