To Simmer

1. (intransitive) To cook or undergo heating slowly at or below the boiling point. [from mid 17th c.]
2. (transitive) To cause to cook or to cause to undergo heating slowly at or below the boiling point.

Synonym: coddle

3. (intransitive, figuratively) To be on the point of breaking out into anger; to be agitated. [from 1760s]
4. (intransitive, figuratively) To remain angry with someone or something past the point of exhaustion; to resign oneself to holding a grudge, especially after some failed attempts to resolve a situation.

simmer down