Origin of the idiom "falling off the wagon"

I often hear the idiom "falling off the wagon", as in "Has Robert Downey Jr. fallen off the wagon?" (i.e. Is he drinking alcohol again?). Where did the phrase originate? What wagon? And why is being "on the wagon" synonymous with being sober?

Solution 1:

From The Encyclopedia of Word and Phrase Origins, by Robert Hendrickson:

The original version of this expression, 'on the water wagon' or 'water cart,' which isn't heard anymore, best explains the phrase. During the late 19th century, water carts drawn by horses wet down dusty roads in the summer. At the height of the Prohibition crusade in the 1890s men who vowed to stop drinking would say that they were thirsty indeed but would rather climb aboard the water cart to get a drink than break their pledges. From this sentiment came the expression 'I'm on the water cart,' I'm trying to stop drinking, which is first recorded in, of all places, Alice Caldwell Rice's Mrs. Wiggs of the Caggage Patch [1901], where the consumptive Mr. Dick says it to old Mrs. Wiggs. The more alliterative 'wagon' soon replaced cart in the expression and it was eventually shortened to 'on the wagon.' 'Fall off the (water) wagon' made its entry into the language almost immediately after its abstinent sister.

Solution 2:

Meaning

World Wide Words explains:

However, the saying is indeed originally American and it is associated with wagons, of a sort. The original form, which dates from the early years of the twentieth century, was to be on the water-wagon, implying that the speaker was drinking water rather than alcohol and so was an abstainer, at least for the time being. The image of the horse-drawn water-wagon would have been an obvious one at the time — it was used to spray unpaved American streets in the dry summer months to dampen down dust thrown up by the traffic. A direct link with the temperance movement — very active at the time — would seem probable, though I’ve not been able to establish this for sure.

Wagon

The OED say on the wagon is originally from the US and has it from a 1906 book by Bert Leston Taylor titled Extra Dry: being further adventures of the Water Wagon:

It is better to have been on and off the Wagon than never to have been on at all.

Water wagon

The OED has on the water wagon from a 1904 Dialect Notes:

‘To be on the water wagon’, to abstain from hard drinks. N.Y.

I found several older examples. The Staunton Spectator and Vindicator (Staunton, Va., May 24, 1901):

Every day we hear men say "I am on the water wagon".

The St. Louis Republic (St. Louis, Mo., January 18, 1903):

Well, I tried to work the gag on a bartender in Cincinnati not long ago. Gammon and I dropped into a quiet place near the theater, and I ordered a high ball.

"I am something of a ventriloquist, as you know, and when I casually asked the dog what he would take, he replied: 'Well, I'm on the water wagon, but I'll take a sandwich.'

The Salt Lake Herald, (Salt Lake City (Utah), December 24, 1903) reports of "An English Blunder":

Charles Warner the English actor tells the following on himself: "A few nights after I reached New York I asked to have a drink. He replied: 'I am on the water wagon.' 'On the what?' I asked. He explained, and I thought it was a good one and resolved to spring it immediately. Two days later I met Mr. Hawtry and he asked me to join him a high ball. 'Sorry, old fellow,' I said, 'but I am on the washtub, don't you know?' Beastly blunder, but very English, was it not?"

Water cart

I'm on the water cart is claimed to have been first recorded in Alice Caldwell Rice's Mrs. Wiggs of the Caggage Patch (1901), but I found it in The Red Cloud Chief (Red Cloud, Webster Co., Neb., July 06, 1900) in "A Tragedy in Slums: Romance in the Low Life of New York":

She visited a few of her friends and confided to them she was going to get married.

"Have a ball?" said one.

"No, I've cut it; I'm on the water cart for good," was the reply.

Solution 3:

Meaning: Abstaining from consumption of alcoholic beverages.

Example: Dean Martin never fell off the wagon. You have to be on the wagon before you can fall off.

Origin: The origin of this seemingly mysterious phrase becomes clear when one learns that the original phrase was “On the water wagon”. A water wagon was a common piece of equipment in the days before paved roads. They were used to spray the dirt roads to help control dust.

Alternative: It dates to Victorian times when prisoners where transported to the Old Bailey on a wagon. The officers guarding them would stop for a drink but the prisoners would have to stay on the wagon.

http://joe-ks.com/phrases/phrasesO.htm

See also: http://www.phrases.org.uk/meanings/on-the-wagon.html