Your local 19th century PhD researcher popping in here to add to this. Toast water is 100% a drink for treating illness. It turns up listed in several household medicine guides in the 19th century, and is listed as for treating people with fever, diarrhoea and vomiting, who can’t keep anything down. It’s essentially oral rehydration therapy.
It interestingly starts turning up in literature in the period covering five major cholera outbreaks in the UK and US (this was obviously an English language Ngram search).
And peaks several times at epidemic peak points (1830s, 1850, 1880s), including its first peak in 1831/2, which corresponds with the first cholera epidemic in the UK.
It also corresponds with the year William Brooke O’Shaughnessy discovered that a lot of people who were dying of cholera were severely lacking water and salts in their blood and urine. Dehydration was found to be a major cause of death in cholera patients. “Toast water” was suggested in the Lancet medical journal in 1832 as an initial treatment for cholera patients.
Most of the recipes in household medicine guides I found suggest sweetening or flavouring the toast water with something if the patient could keep it down in order to cover the terrible taste.
People in the past were just people. And in this particular case, they were trying to keep their loved ones from dying of cholera.