If someone or something quietens down or if you quieten them down, they become less noisy or less active. (mainly BRIT; in AM, usually use quiet down )
The labour unrest which swept the country last week has quietened down...
Somehow I managed to quieten her down...
Tom’s words before the match might also have quietened down our own supporters.
= calm down
PHRASAL VERB: V P, V n P, V P n (not pron)