freemason (n.)

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 was continued into or revived in the 17th century and by 1717 had developed into the secret fraternity of affiliated lodges known as Free and Accepted Masons (commonly abbreviated F. and A. M.). The accepted refers to persons admitted to the society but not belonging to the craft.

The exact origin of the free- is a subject of dispute. Some [such as 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]. Related: freemasonic.