Definition of quiet noun from the Oxford Advanced American Dictionary

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quiet

noun
/ˈkwaɪət/
 
 
[uncountable] the state of being calm and without much noisethe quiet of his own roomthe quiet of the early morningI go to the library for a little peace and quiet.IDIOM

on the quiet

without telling anyone
synonym secretlyHe'd been meeting her on the quiet.on the quiet