sicker than a dog?

[marcycma]

Where did the phrase sicker than a dog originate?

[maxi29]

Proverbs (26 v11) says that "As a dog returneth to its own vomit, so a fool returneth to his folly".

I don't know if that's the reason for the phrase but perhaps set the example of an animal renowned for vomiting.

[Quizmonster]

The earliest recorded use of the phrase 'sick as a dog' is dated 1705. At much the same time, people were using other phrases such as 'sick as a horse' and even - believe it or not - 'sick as a cushion'! More recently, we seem to have discovered, especially if we happen to be football-players, the phrase, 'sick as a parrot'. It is all just part of the rich tapestry of English usage.