What do you mean by serious? Third grade.

This word means a classic that conforms to moral norms.

After "serious", it is used to describe a serious and serious attitude, sometimes with irony.

This idiom can be used to describe people who talk and do things seriously, without joking or talking casually. For example, you can say, "He is always serious and unsmiling." This sentence means that he is always serious at ordinary times, and he doesn't tell jokes casually or say unimportant things.