This word is one of the oldest, as it is one of the most beautiful, in any language. Nobody knows where or when it originated, but it is certain that it existed in the Sanskrit, in a form strikingly similar to that used by us. In Greek it was phrater, in the Latin frater, whence our "fraternal" and "fraternalism." It has always meant men from the same parents, or men knit by very close blood ties. When associated with "initiation, which las the general meaning of "being born into," one can see how appropriate is its k use in Freemasonry. All of us have, through initiation in our "mother" Lodges, been born into a Masonry and therefore we are "brothers," and that which holds us together in one great family is the "Mystic Tie," the Masonic analogue of the blood tie among kinsmen. Or the chain of union in the European tradition.
In Masonic dictionary.