1
general::
Become calm after anger or excitement, as in Simmer down, Mary; I'm sure he'll make it up to you, or I haven't time to look at your report now, but I will when things have simmered down a bit. This idiom derives from simmer in the sense of "cook at low heat, below the boiling point." [Second half of 1800s]
American Heritage Idioms
2
general::
Phrase(s): simmer down
1. Lit. to decrease in intensity. (As boiling dies down when the heat is lowered or removed.) • The hectic activity of the day finally simmered down. • When things simmer down in the fall, this is a much nicer place. 2. Fig. [for someone] to become calm or less agitated. • I wish you would simmer down. • Please simmer down, you guys!
McGrawhill's American Idioms And Phrasal Verbs