When someone is sicks, they are not feeling well or healthy.
Being sicks is not good because it means you might need to rest or see a doctor.
Examples:
She feels sicks and needs to stay in bed.
He looks sicks, so he should see the doctor.
The sicks dog needs to go to the vet.
The sicks cat sneezes a lot.
I feel sicks when I eat too much candy.
The sicks bird doesn't sing today.