Story goes that in the 18th century, people were trying to find out whether alcohol was pure or not. It was discovered that an alcohol and water solution mixed with gun powder will still light when a flame is put to it; therefore this was 100% proof that it was good alcohol. If it did not light, it was considered “under-proof” and not good alcohol.
This proven amount is now know to be 57.15% ethanol. For ease of measurement, most people consider the proof amount to be double what the percent of alcohol is. Therefore, a 80 proof alcohol is approximately 40% alcohol.