Definition of simmers:
(v) :
(intransitive) To cook or undergo heating slowly at or below the boiling point.
(v) :
(transitive) To cause to cook or to cause to undergo heating slowly at or below the boiling point.
(n) :
The state or process of simmering.
(v) :
(intransitive, figurative) To be on the point of breaking out into anger; to be agitated.
(v) :
(intransitive, figurative) 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.
(v) :
(intransitive, figurative) To develop gradually, of an idea or plan.