means To cook something gently in liquid kept just below boiling, where small bubbles rise lazily rather than churning — or, by extension, to keep something (anger, tension, an idea) quietly active beneath the surface.

from From the earlier English verb 'simper' — not the coy smile, but a separate word meaning 'to boil gently' — which softened into 'simmer' by the 17th century. The shift from a hard 'p' to a humming 'm' is itself onomatopoeic: 'simper' snaps, while 'simmer' murmurs, mimicking the low steady sound of a pot held just shy of a boil. The deeper roots are uncertain, but the word is likely imitative — language doing its best impression of a quiet, sustained heat.