serious /ˈsɪə̯.ɹi.əs/adjective 4adverb 1forms 4 adjective4 •not smiling or making jokes, and looking thoughtful rather than happyIt was a surprise to see the captain, who had always seemed so serious, laugh so heartily. •Important; weighty; not insignificant.This is a serious problem. We'll need our best experts. •Really intending what is said (or planned, etc); in earnest; not jocular or deceiving.After all these years, we're finally getting serious attention.He says he wants to buy the team, but is he serious? •Committed. adverb1 •(dialect, informal) seriously, in a serious manner (most often heard in take or mean serious)The only time I walk out on singin' is when there's muckin' about and youse don't take it serious. — 1957 formsmore serious · seriouser · most serious · seriousest