Why is someone with a lot of nerve referred to as being “full of moxie”?

Today Moxie is a New England soft drink, but it began as a tonic
invented by Dr. Augustine Thompson in 1884 as “Moxie Nerve Food.”
Although the 1906 Pure Food and Drug Act put an end to its medicinal
claims, there are still those who say Moxie gives them energy, and so
to be “full of moxie” means to be full of false nerve.