Simmer (verb)

1. Cook or heat (something) slowly and gently, just below boiling point.
2. Be in a state of suppressed anger or annoyance.

Origin:

Late 16th century (as a noun denoting a low, gentle boil): from old norse sýma "be calm".

Examples:

1. She simmered the soup for an hour to allow the flavors to develop.
2. The pot of water simmered gently on the stove.
3. The anger simmered within him for months before he finally exploded.
4. Her frustrations simmered as she sat through the long meeting.
5. The conflict simmered for years until it finally boiled over.