Where does the phrase sick as a dog come from?

Answer 1

sick dogs are more noticable then sick people most of the time

There are several expressions of the form sick as a ..., that date from the eighteenth and nineteenth centuries. Sick as a dog is actually the oldest of them, recorded from 1705; it is probably no more than an attempt to give force to a strongly worded statement of physical unhappiness. It was attached to a dog, I would guess, because dogs often seem to have been linked to things considered unpleasant or undesirable; down the years they have had an incredibly bad press, linguistically speaking (think of dog tired, dog in the manger, dog's breakfast, go to the dogs, dog Latin - big dictionaries have long entries about all the ways that dog has been used in a negative sense).

At various times cats, rats and horses have been also dragged in to the expression, though an odd thing is that horses can't vomit; one nineteenth-century writer did suggest that this version was used "when a person is exceedingly sick without vomiting". The strangest member of the set was used by Jonathan Swift in 1731: "Poor Miss, she's sick as a Cushion, she wants nothing but stuffing" (stop laughing at the back).

The modern sick as a parrot recorded from the 1970s - at one time much overused by British sportsmen as the opposite of over the moon - refers to a state of deep mental depression rather than physical illness; this perhaps comes from instances of parrots contracting psittacosis and passing it to their human owners.