simmer

• process of being cooked at a temperature just below the boiling point; state of being cooked at a temperature just below the boiling point
cook steadily at a temperature just below the boiling point; be steadily cooked at a near-boiling temperature; seethe, be in a state of suppressed anger or excitement
when you simmer food, or when it simmers, you cook it by keeping it just below boiling point.
when a violent situation or quarrel is simmering over a period of time, it is not openly expressed, but is likely to break out suddenly.
if you simmer down, you stop being angry about something; an informal expression.