serious

Without humor or expression of happiness; grave in manner or disposition.

deadly serious

It 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.

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.