An idiom meaning to do or say something that makes a bad situation worse (equivalent to 'add fuel to the fire').
そんな批判を公にすると、ただ火に油だよ。
Making that criticism public will only add fuel to the fire.
彼の挑発は争いを火に油のように激化させた。
His provocation intensified the dispute like adding fuel to the fire.
状況が緊迫しているときに不要な一言は火に油だ。
When tensions are high, an unnecessary remark just adds fuel to the fire.