knowyourphrase.com - The Meanings and Origins For Several Common Idioms and Phrases
The word "two" in the phrase refers to things that have been finished, while the word "one" references that which is yet to be finished. Example: If there were three cookies, and I ate two of them, then I could say that two were down, and I had one to go.
This phrase may come from sports. The reason is because the earliest writings of the phrase that I could come from the first thalf of the 20th century, and it's pretty much always used in the context of sports.
This term is applicable to different sports. For example, in baseball, the word "down" can be synonymous for "out." A batter or baserunner can be ruled as "out" by an umpire. If the team on defense gets three offensive players out, then the teams switch places. Hence, sometimes you might hear that the offensive team has two players down, and one to go.
The expression, used in relation to baseball, can be seen in the Olean Evening Herald, June 1922, where a recent game had finished and a recap of it was given:
"The visting Stars again tied the score in the seventh with two down and one to go."
The phrase is also used in the sport of golf, appearing in Des Moines Daily News, June 1919, to describe the number of holes left:
"In defeating Walker Jr., youthful star from Staten Island was pushed to the utmost reaching the
final hole of the day with the youngster two down, one hole to go."
Reference: The Facts on File Encyclopedia of Word and Phrase Origins
* I'm attempting to go three weeks without eating any sweets, and so far I have two down and one more week to go.
Note: A phrase's origins are, much of the time, completely unclear. The origins you see listed are the plausible theories floating around for how or where an idiom came from, but may not necessarily be accurate. The quotes which have the phrase in them are the oldest written forms of the phrase I could find, but keep in mind that older recordings are probably out there somewhere. If you find an earlier citation than what I have, feel free to let me know!
Also, remember that just because you see a saying in an old book or newspaper, let's say from the year 1893, it does not mean the saying originates from that source. In all likelihood, if an expression is already in use in a book or newspaper, then it's probably older. Nevertheless, these old quotes serve as a way to show the reader how far back in history some of these sayings go.