The story
From green + cloth, from the tablecloth of green baize that covered the table at which its members sat.
Etymology from Wiktionary, CC BY-SA 4.0.
greencloth
Today it means: A board or court of justice formerly held in the counting-house of the British sovereign's household, composed of the Lord Steward and his officers, and having cognizance of matters of justice in the household, with power to correct offenders and keep the peace within the vicinity of the palace. Full definition →