1 seethe, stew, cook, boil, bubble:
A huge cauldron of soup simmered on the fire.
2 chafe, seethe, stew, steam, smoulder, fume, rage, burn, Colloq US do a slow burn:
Father is still simmering because you scratched his car.
3 simmer down. calm or cool down, cool off, calm oneself, become quiet, control oneself, get control of or over oneself, Chiefly Brit quieten down, Slang cool it:
Yesterday he was in a towering rage, but he has simmered down a bit today.