Fig.: to vomit a lot

I love mushrooms, but the one time I had a cold salad of porcini mushrooms at the restaurant, I got as sick as a dog all the way home and all night

Adoro I funghi, ma quell'unica volta che ho mangiato al ristorante un'insalata fredda di porcini, sono stata da cani fino a casa e per tutta la notte seguente

Poor Irene, who gets seasick so easily, was sick as a dog throughout the entire crossing while on the little boat that was taking her to the island of her dreams

La povera Irene, che soffre il mal di mare, ha vomitato l'anima per tutta la traversata sulla barchetta che la portava alla sua isola di sogno

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”