simmering

To cook or undergo heating slowly at or below the boiling point.

“The soup simmered on the stove.”

To cause to cook or to cause to undergo heating slowly at or below the boiling point.

“Simmer the soup for five minutes, then serve.”

To be on the point of breaking out into anger; to be agitated.

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.

“I tried to get through to him; all that's left for me to do is simmer.”

The act of something that simmers.