“Moxie,” used generally from 1930, comes from the brand name of a bitter syrup first marketed as the medicine “Moxie Nerve Food” in 1876, then sold as a soft drink starting in 1884. The brand may be from a Native American Abenaki word for “dark water,” from Maine lake and river names.
The beverage Moxie, originally patented as medicine, is still sold today by the Moxie Beverage Company in Bedford, NH. It was one of the first mass-produced soft drinks in the US, arising during a boom in soda brands that included Dr. Pepper (which was notably served at the 1885 Louisiana Purchase Exposition), as well as other now-defunct ginger ale and root beer brands.
Moxie’s creator, Dr. Augustin Thompson, sought to create a medicine that did not contain then-common but potentially harmful drug ingredients such as cocaine and alcohol. Moxie was sold as the “nerve food” syrup first, marketed as a remedy for “paralysis, softening of the brain, nervousness, and insomnia,” and later the syrup would be sold for use in soda fountains.
At the time he developed Moxie, Thompson claimed that the bitter extract used to make the syrup was from a rare and unnamed South American plant, but it was later determined to be gentian root extract, a fairly common substance that has been used in tonics since at least 170 BC during the reign of Gentius, namesake of the plant’s genus gentiana and the last king of the powerful Ardiaei tribe in Illyria.
Thompson also claimed that he named Moxie after his friend Lieutenant Moxie, who discovered the plant and its extract, but it’s likely that no such person existed and that he drew the name from a few rivers and lakes in Maine, where Thompson was born. These bodies of water included variations of the word “moxie,” meaning “dark water” in Eastern Abenaki languages, which was and is spoken by the Mi’kmaq, Maliseet, Passamaquoddy and Penobscot people of coastal Maine.
The current, most common usage of the word, referring to bold determination and spunk, comes from the gist of the marketing for the medicine (and then beverage), which said that it would “build up your nerve.”
Useless Etymology 