To be very sick.
to be in a state that is very unpleasant.
Sick as a horse (when one is sick without the sensation of vomiting).
You should not go on holiday with him. He seemed to be as sick as a dog, according to his doctor.
I don't like to be as sick as a dog, that is why I am taking care about myself.
There is no origin for this idiom
Common phrase that has a specific meaning.