Fig.: to be miserably sick.
(informal)

I caught that new Australian flu and I've been as sick as a dog for three whole days with a very high fever

Mi sono beccato la nuova influenza australiana e sono stato male da cani per tre giorni interi con febbre altissima

When spring rolls around, I become a wreck because of all my allergies

Quando arriva la primavera sto sempre da cani con tutte le mie allergie

"I heard you've been kind of sick the past few days" "Kind of?!? I was as sick as a dog!"

“Ho saputo che non sei stato bene nei giorni scorsi” “Non sono stato bene?!? Sono stato da cani!”

The original meaning of this phrase referred to vomiting, and it's easy to see how often dogs eat what they shouldn't and get sick. Most etymologists agree that it dates back to the early 1700s, although it seems to have first appeared in print in 1592 in Harvey's Works: "Now sicke as a dog"