Every once in a while you may turn the pages of Reg Rom and see the phrase “sick as a horse”.
From the Lexicon Balatronicum (1811): “Horses are said to be extremely sick at their stomachs from being unable to relieve themselves by vomiting. Bracken, indeed, in his Farriery gives an instance of that evacuation being procured but by a means which he says would make the Devil vomit. Such as may have occasion to administer an emetic either to the animal or the fiend may consult his book for the recipe.”
In other words, nausea.
Curious that the more popular phrase in the modern era is “healthy as a horse”, although it appears to be an expression most popular in the US that began making its appearance around the 1860s.
