Merriam-Webster's Collegiate® Dictionary, 11th Edition
SIMMER
I. verb (simmered; simmering)
Etymology: alteration of English dialect simper, from Middle English simperen, of imitative origin
Date: 1653
intransitive verb to stew gently below or just at the boiling point,
2. to be in a state of incipient development ; ferment , to be in inward turmoil ; seethe , transitive verb to cook slowly in a liquid just below the boiling point,
II. noun
Date: 1809
the state of simmering