This is the 666th most frequent Vietnamese word.
nghiêm
"Strict" or "serious."
Here, the word "nghiêm" (part of "nghiêm túc") indicates seriousness or strictness in attitude or demeanor.
Anh ta có thái độ nghiêm túc trong công việc.
He has a serious attitude towards work.
Here, "nghiêm chỉnh" means adhering to a code of conduct rigorously or with discipline.
Cô giáo yêu cầu học sinh giữ trật tự trong giờ học một cách nghiêm chỉnh.
The teacher requested the students to maintain order during class strictly.
In this sentence, "nghiêm túc" conveys the sense of carrying out actions with seriousness and adherence to rules.