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Synonyms for Quieten (same or very similar meaning)

WordNet sense 1 (become quiet or quieter):
hush, quiet, quiesce, pipe down, quiet down

WordNet sense 2 (make calm or still):
calm, lull, still, tranquilize, tranquillize, tranquillise, calm down

WordNet sense 3 (cause to be quiet or not talk):
silence, shut up, hush up

From the ODE community, based on WordNetadd/edit