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late 14c., originally a traveling guild of masons with a secret code; in the early 17c. they began accepting honorary members and teaching them the secrets and lore, which by 1717 had developed into the fraternity of Free and Accepted Masons.
The exact origin of the free- is a subject of dispute. Some [e.g. Klein] see a corruption of French frère "brother," from frèremaçon "brother mason;" others say it was because the masons worked on "free-standing" stones; still others see them as "free" from the control of local guilds or lords [OED].