an iron fist in a velvet glove

Từ điển Oxford Advanced Learner 8th

an iron ˈfist/ˈhand (in a velvet ˈglove) idiom
if you use the words an iron fist/hand when describing the way that sb behaves, you mean that they treat people severely. This treatment may be hidden behind a kind appearance (the velvet glove)
• They promised that the army would strike with an iron fist at any resistance.
•The iron hand in the velvet glove approach seems to work best with this age group.
Main entry: ↑iron_idiom