Etymology From Middle English seryows, from Old French serieux, from Medieval Latin sēriōsus, an extension of Latin sērius (“grave, earnest, serious”), possibly ultimately from Proto-Indo-European *swer- (“heavy”). (If this root is correct, cognate with German schwer (“heavy, difficult, severe”), Old English swǣr (“heavy, grave, grievous”). More at swear, sweer. However, see sērius for an alternative derivation of the Latin.)
1
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.
2
Important; weighty; not insignificant.
This is a serious problem. We'll need our best experts.
3
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?
Etymology From Middle English seryows, from Old French serieux, from Medieval Latin sēriōsus, an extension of Latin sērius (“grave, earnest, serious”), possibly ultimately from Proto-Indo-European *swer- (“heavy”). (If this root is correct, cognate with German schwer (“heavy, difficult, severe”), Old English swǣr (“heavy, grave, grievous”). More at swear, sweer. However, see sērius for an alternative derivation of the Latin.)
1
Colloquial, Dialectal 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, Ray Lawler, Summer of the Seventeenth Doll, Sydney: Fontana Books, published 1974, page 68: