Serious (adjective)

1. Important and requiring attention or thought.
2. Not joking or not to be taken lightly.
3. Having a solemn and dignified manner.

Origin:

Middle english seriouse, from old french serieus, from latin serius early, grave, from serere to join, arrange.

Examples:

1. The matter is serious and requires immediate attention.
2. He was serious when he said he would not attend the party.
3. She has a serious look on her face.
4. The situation is serious and requires a serious response.
5. It's a serious offense and requires a serious punishment.