Learner's definition of SERIOUS
[more serious; most serious]
1 : having an important or dangerous possible result
a serious injury/illness/condition
“You sound terrible.” “It's just a bad cold. Nothing serious.”
For my brother, not going to college was a serious mistake.
Crime is a serious problem in this neighborhood.
2 : involving or deserving a lot of thought, attention, or work
a serious study
The team is a serious contender for the championship.
If you want to quit smoking, you have to make a serious effort.
They had a serious conversation about their relationship.
a serious novel about modern life
The story raises serious questions about our system of justice.
Dog shows are a serious business.
3 : giving a lot of attention or energy to something
She is a serious cyclist who rides 200 miles each week.
Serious journalists do not pry into the personal lives of famous people.
When it comes to fishing, he's very serious. = He's very serious about fishing.
My sister is serious about her grades.
4 : not joking or funny
a serious story/opera
Don't laugh. I'm serious!
Can you be serious just this once? This is important.
He is serious about moving down South. [=he truly wants/intends to move down South]
“He says he won't do it.” “Are you serious?”
“I'm joining the army.” “You can't be serious.” [=you have to be joking; I can't believe that you're really joining the army]
5 always used before a noun, informal : large or impressive in quality or amount
After business school, he started making some serious money. [=he started making a large amount of money]
These are serious shoes!
They did some serious drinking at the bar. [=they drank a lot]
6 : having or involving strong romantic feelings
— often + about
Their romance is quite serious. They're even talking about marriage.
We've dated a few times, but it's nothing serious.
They are a serious couple.
He's never really had a serious relationship with a woman.
7 : thoughtful or quiet in appearance or manner