v.
  - stay or cause to stay just below boiling point while bubbling gently.
 - be in a state of suppressed anger or excitement. 
▸
(simmer down)
 become calmer and quieter.
 
n.
 a state or temperature just below boiling point.
 
  Etymology
  C17: alt. of dial. simper (in the same sense), perh. imitative.