simmer

Concise Oxford English Dictionary © 2008 Oxford University Press:
simmer/ˈsɪmə(r)/

▶verb

• 1 stay or cause to stay just below boiling point while bubbling gently.

• 2 be in a state of suppressed anger or excitement.

  ■ (simmer down) become calmer and quieter.

▶noun a state or temperature just below boiling point.

– origin C17: alt. of dial. simper (in the same sense), perh. imitative.