Soothe or calm down something or someone, as in The twins are quarreling so I'd best go pour oil on troubled waters. This term alludes to an ancient practice of pouring oil on ocean waves to calm their turbulence, which was mentioned in the eighth century. [Mid-1800s]

To calm a disturbance: “His ideas caused real dissension within the party at first, but he poured oil on troubled waters in last night's speech.”