After I was corrected, I looked up the origin of the word. It first appeared as the name of a character in a comic strip, "Caspar Milqutoast" who the author described as "speaks quietly and gets hit with a big stick." He named it after the food which is the most meek of meals.

The Wikipedia says this: "The character's name is derived from a bland and fairly inoffensive food, milk toast, which, light and easy to digest, is an appropriate food for someone with a weak or "nervous" stomach"

I never knew that this comic was a thing, but now I want to read it