Sorry I cannot source this but my Grandma(?) told me that the saying meant that if you feed someone with a cold, it will get worse and they will develop a fever. Then you will have to starve him for sure. Can anyone confirm or correct this?
Maybe it’s just one of those old sayings that you take as is, and can’t really be explained to someone who does not.
i.e. One of my sisters has never understood the saying:
“You can have your cake and eat it too!”
I tried explaining it to her, but she never did get it.
I finally told her she was overthinking it.
From Scientific American: “This saying has been traced to a 1574 dictionary by John Withals, which noted that “fasting is a great remedy of fever.” The belief is that eating food may help the body generate warmth during a ‘cold’ and that avoiding food may help it cool down when overheated”. So it sounds like your Grandma was passing down the wisdom as it was originally meant!