no-go area

Từ điển Oxford Advanced Learner 8th

ˌno-ˈgo area [no-go area no-go areas] BrE NAmE noun (especially BrE)
an area, especially in a city, which is dangerous for people to enter, or that the police or army do not enter, often because it is controlled by a violent group: (figurative)Some clubs are no-go areas for people over 30.
• (figurative)This subject is definitely a no-go area (= we must not discuss it).