My mistake for not being clearer that I had heard the word, understanding that it was both a phrase and a soda brand, before reading BotR. Mad magazine used both meanings in its humor - the term in the dialogue of comic book characters in various parodies, and the soda in the illustrations. But other pulp-style writers from the 40s and 50s used the term too, evoking what seemed like New York City slang dialogue; and being from New England I did see the soda occasionally as I got older. I actually assumed that the soda was named after the phrase, i.e., the soda has moxie or gives you moxie; but I now wonder, as I said, which use of the word actually came first.