Milquetoast was a word that I first encountered in a comic strip, though not the one this blog post is focused on. In Berkeley Breathed's strip Bloom County, and its sequel strip Outland, there was a character named Milquetoast, who was apparently a cockroach, though he didn't look much like one. I remember being ignorant of how to pronounce his name, though I didn't really give it much thought as I figured it was just a weird name Breathed had come up with.
It was certainly a strange joke word created by a cartoonist, but not one coined by Breathed.
The word first appeared in 1924, in a comic strip called The Timid Soul by H.T. Webster. Webster had been doing single panel gag strips for the New York Tribune since 1912. His panels appeared under a number of different recurring titles, which would change depending on the subject matter of the gag, but none of them had any recurring characters. In 1924, he moved to the New York World, and while he did continue his other panels under their various other titles, he began a new one, The Timid Soul, and with it created a character by the name of Caspar Milquetoast. Caspar was, as the title of the strip suggests, the most meek and timid man you would ever meet, and his meekness would always land him in strange predicaments. Many of them involve him strictly following signs, such as when he decides to buy a new hat rather than retrieve one that has blown off his head and landed next to a "Keep Off The Grass" sign, or when he goes to the river for a dip and lights a cigar, but drops it when a discarded "No Smoking" sign floats by, or when he is completely immobilized by a "Watch This Space" sign. Many others involve him doing his best to avoid confrontations, whether it be not talking to someone about how he thinks the Dodgers are going to do this year, or not contradicting his neighbor when the neighbor holds him responsible for a hurricane blowing a tree onto his fence, or stopping by the side of the road to change his tie so he wouldn't have to tell his wife he hates the one she got him for Christmas.
Webster's The Timid Soul strips became his most popular, and Caspar Milquetoast literally became a household name. Comparisons of various people to Caspar in the wider media began quite early, but the reference was always to the comic strip character. Someone might be called "a Caspar Milquetoast," or "a Mr. Milquetoast," or even "a Milquetoast," but it was always a proper noun. Sometime around the early to mid-1930s it dropped the capital "M," and also began its life as an adjective, which is the form in which it's most likely to be found these days. In 1988 Berkeley Breathed created a cockroach character with that name, which I now realize was meant to be ironic.
The word itself is confusing to many people due to their ignorance of its origins. Part of it looks vaguely French, so one might assume it came from French or Latin somehow, though that wouldn't explain the "toast" at the end, which is very English looking. It wouldn't occur to most people that a funny looking word with no discernible linguistic origin may have been invented for a comic strip. The word does have a clear etymology outside of just being a comic strip character's name, however: Webster was obviously referencing "milk toast," a dish reserved for those with weak stomachs, and possibly thinking of the term "milksop," which references a similar dish, and has been used for a few centuries to refer to weak, timid men. In fact, it's possible he may have gotten the two dishes confused when concocting his comic character, though that's just speculation on my part.
For more information:
Hairy Green Eyeball has a nice collection of The Timid Soul strips
WordOrigins.org for more on the origin of the word
Atlas Obscura for more on the character and its creator
Neglected Books for more on H.T. Webster