From Wiktionary, the free dictionary
trúng (“to hit; to be right on”) + tủ (“cabinet; highly useful skill or knowledge; high-yield topic”).
trúng tủ
- (colloquial, of the examinee) to be fortunate and be tested on the only thing(s) one has studied
- (rare) to be right on by saying things that someone else had tried to keep to themselves and did not want others to know