3 meaning what you say or do, and not making a joke
I’m sorry, I didn’t realize you were being serious.
The police have made no serious attempt to address these issues.
serious about: I’m not convinced they’re serious about the deal.
serious about (doing) something: Do you think Mike’s serious about going to live in New York?
deadly/dead (=completely) serious: “Are you serious about leaving your job?” “Dead serious.”
Synonyms and related words