hard nut

Từ điển Oxford Advanced Learner 8th

▪ I. a hard/tough ˈnut idiom
(informal)a person who is difficult to deal with or to influence
Main entry: ↑nut_idiom
 
▪ II. a hard/tough ˈnut (to ˈcrack) idiom
a difficult problem or situation to deal with
Main entry: ↑nut_idiom