Probably imitative of gentle bubbling, altered from Middle English 'simperen'—filling a gap for cooking below boiling.
To cook liquid at just below boiling point, with small bubbles rising gently to the surface.
An alteration of Middle English 'simperen' (to simmer), possibly imitative of the quiet bubbling sound. Some scholars connect it to a Low German source. The word captured a precise cooking distinction—below boiling but above warming—that had no prior English term. Key roots: simperen (Middle English: "to simmer (possibly imitative)").
A simmer is precisely 85–95°C (185–205°F). Before thermometers, cooks described it as 'smiling water'—tiny bubbles that look like dimples on the surface.