The rue au Beurre is one of the city's oldest streets emerging as a lane called Santstraete ("Sand Street") probably because it followed the course of a sandbank in the marshy soil. The street linked the church of Saint-Nicholas with the Grand' Place. At the market in front of the church all types of goods were initially sold, but butter came to predominate by the 15th century as cloth makers were forbidden to employ any other substance to grease their looms. The market lasted until 1798 when it was moved to the area around the current Bourse. The rue au Beurre—so named from the 15th century—was called officially the Grande rue au Beurre in the 18th and 19th centuries to distinguish it from the Petite rue au Beurre, which was laid out about the 18th century.
Historical Dictionary of Brussels. Paul F. State.