Phrasal verbs, also called two-word verbs, are idiomatic expressions wherein the second element of the verb (the adverb or particle) is not necessarily predictable.
Phrasal Verb:
simmer down
Meaning / Example:
1. to decrease in intensity. (As boiling dies down when the heat is lowered or removed.) > When things simmer down in the fall, this is a much nicer place.
2. [for someone] to become calm or less agitated. > I wish you would simmer down.