padded cell
US /ˌpæd.ɪd ˈsel/
UK /ˌpæd.ɪd ˈsel/

1.
a room with soft walls in a mental hospital, for violent patients
:
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The agitated patient was placed in a padded cell for their safety.
•
The doctor recommended a padded cell for the patient exhibiting extreme self-harming behavior.